MXS-1354: Prevent removal of last admin account

Removing the last admin account is now forbidden. This should prevent most
cases where users could lock themselves out of the administrative
interface.

This change does allow a non-root network user to be the last admin
account. In practice this does not prevent the root user from gaining
access to maxadmin. Access can be gained by removing the users file and
restarting MaxScale or by editing the users file by hand.

include/maxscale/users.h

@@ -109,13 +109,13 @@ bool users_find(USERS* users, const char* user);
 bool users_is_admin(USERS* users, const char* user);
 
 /**
- * Check if at least one admin account exists
+ * Check how many admin account exists
  *
  * @param users Users to check
  *
- * @return True if at least one admin account exists
+ * @return Number of admin accounts
  */
-bool users_have_admin(USERS* users);
+int users_admin_count(USERS* users);
 
 /**
  * Dump users as JSON