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/modules/authenticator/max_admin_auth.c

@@ -142,7 +142,7 @@ max_admin_auth_set_protocol_data(DCB *dcb, GWBUF *buf)
         dcb->data = (void *)session_data;
 
         /* Check for existance of the user */
-        if (admin_local_search_user(session_data->user))
+        if (admin_linux_account_enabled(session_data->user))
         {
             session_data->validated = true;
             return 0;