Allow socket and address/port to be used with maxadmin

It's now possible to use both a Unix domain socket and host/port
when connecting with MaxAdmin to MaxScale.

By default MaxAdmin will attempt to use the default Unix domain
socket, but if host and/or port has been specified, then an inet
socket will be used.

maxscaled will authenticate the connection attempt differently
depending on whether a Unix domain socket is used or not. If
a Unix domain socket is used, then the Linux user id will be
used for the authorization, otherwise the 1.4.3 username/password
handshake will be performed.

adminusers has now been extended so that there is one set of
functions for local users (connecting locally over a Unix socket)
and one set of functions for remote users (connecting locally
or remotely over an Inet socket).

The local users are stored in the new .../maxscale-users and the
remote users in .../passwd. That is, the old users of a 1.4
installation will work as such in 2.0.

One difference is that there will be *no* default remote user.
That is, remote users will always have to be added manually using
a local user.

The implementation is shared; the local and remote alternatives
use common functions to which the hashtable and filename to be
used are forwarded.

The commands "[add|remove] user" behave now exactly like they did
in 1.4.3, and also all existing users work out of the box.

In addition there is now the commands "[enable|disable] account"
using which Linux accounts can be enabled for MaxAdmin usage.

server/core/test/testadminusers.c

@@ -49,12 +49,12 @@
 static int
 test1()
 {
-    if (admin_remote_verify("admin", "mariadb") == 0)
+    if (admin_verify_inet_user("admin", "mariadb") == 0)
     {
         fprintf(stderr, "admin_verify: test 1.1 (default user) failed.\n");
         return 1;
     }
-    if (admin_remote_verify("bad", "user"))
+    if (admin_verify_inet_user("bad", "user"))
     {
         fprintf(stderr, "admin_verify: test 1.2 (wrong user) failed.\n");
         return 1;
@@ -75,13 +75,13 @@ test2()
 {
     const char *err;
 
-    if ((err = admin_local_add_user("user0")) != NULL)
+    if ((err = admin_enable_linux_account("user0")) != NULL)
     {
         fprintf(stderr, "admin_add_user: test 2.1 (add user) failed, %s.\n", err);
 
         return 1;
     }
-    if (admin_local_add_user("user0") == NULL)
+    if (admin_enable_linux_account("user0") == NULL)
     {
         fprintf(stderr, "admin_add_user: test 2.2 (add user) failed, duplicate.\n");
 
@@ -89,7 +89,7 @@ test2()
     }
 
     /* Deleting the last user is not forbidden so we expect this to succeed */
-    if ((err = admin_local_remove_user("user0")) != NULL)
+    if ((err = admin_disable_linux_account("user0")) != NULL)
     {
         fprintf(stderr, "admin_remove_user: test 2.3 (add user) failed, %s.\n", err);
 
@@ -97,7 +97,7 @@ test2()
     }
 
     /* Add the user back, for test5. */
-    if ((err = admin_local_add_user("user0")) != NULL)
+    if ((err = admin_enable_linux_account("user0")) != NULL)
     {
         fprintf(stderr, "admin_add_user: test 2.4 (add user) failed, %s.\n", err);
 
@@ -121,35 +121,35 @@ test3()
 {
     const char *err;
 
-    if ((err = admin_local_add_user("user1")) != NULL)
+    if ((err = admin_enable_linux_account("user1")) != NULL)
     {
         fprintf(stderr, "admin_add_user: test 3.1 (add user) failed, %s.\n", err);
 
         return 1;
     }
 
-    if (admin_local_search_user("user1") == 0)
+    if (admin_linux_account_enabled("user1") == 0)
     {
         fprintf(stderr, "admin_search_user: test 3.2 (search user) failed.\n");
 
         return 1;
     }
 
-    if (admin_local_search_user("user2") != 0)
+    if (admin_linux_account_enabled("user2") != 0)
     {
         fprintf(stderr, "admin_search_user: test 3.3 (search user) failed, unexpeted user found.\n");
 
         return 1;
     }
 
-    if ((err = admin_local_remove_user("user1")) != NULL)
+    if ((err = admin_disable_linux_account("user1")) != NULL)
     {
         fprintf(stderr, "admin_remove_user: test 3.4 (add user) failed, %s.\n", err);
 
         return 1;
     }
 
-    if (admin_local_search_user("user1"))
+    if (admin_linux_account_enabled("user1"))
     {
         fprintf(stderr, "admin_search_user: test 3.5 (search user) failed - user was deleted.\n");
 
@@ -179,7 +179,7 @@ test4()
     for (i = 1; i < n_users; i++)
     {
         sprintf(user, "user%d", i);
-        if ((err = admin_local_add_user(user)) != NULL)
+        if ((err = admin_enable_linux_account(user)) != NULL)
         {
             fprintf(stderr, "admin_add_user: test 4.1 (add user) failed, %s.\n", err);
 
@@ -190,7 +190,7 @@ test4()
     for (i = 1; i < n_users; i++)
     {
         sprintf(user, "user%d", i);
-        if (admin_local_search_user(user) == 0)
+        if (admin_linux_account_enabled(user) == 0)
         {
             fprintf(stderr, "admin_search_user: test 4.2 (search user) failed.\n");
 
@@ -201,7 +201,7 @@ test4()
     for (i = 1; i < n_users; i++)
     {
         sprintf(user, "user%d", i);
-        if ((err = admin_local_remove_user(user)) != NULL)
+        if ((err = admin_disable_linux_account(user)) != NULL)
         {
             fprintf(stderr, "admin_remove_user: test 4.3 (add user) failed, %s.\n", err);
 
@@ -223,14 +223,14 @@ test5()
 {
     const char *err;
 
-    if ((err = admin_local_add_user("user")) != NULL)
+    if ((err = admin_enable_linux_account("user")) != NULL)
     {
         fprintf(stderr, "admin_add_user: test 5.1 (add user) failed, %s.\n", err);
 
         return 1;
     }
 
-    if ((err = admin_local_remove_user("user0")) != NULL)
+    if ((err = admin_disable_linux_account("user0")) != NULL)
     {
         fprintf(stderr, "admin_remove_user: test 5.2 (add user) failed, %s.\n", err);