Use RFC 3986 compliant addresses in log messages

When log messages are written with both address and port information, IPv6
addresses can cause confusion if the normal address:port formatting is
used. The RFC 3986 suggests that all IPv6 addresses are expressed as a
bracket enclosed address optionally followed by the port that is separate
from the address by a colon.

In practice, the "all interfaces" address and port number 3306 can be
written in IPv4 numbers-and-dots notation as 0.0.0.0:3306 and in IPv6
notation as [::]:3306. Using the latter format in log messages keeps the
output consistent with all types of addresses.

The details of the standard can be found at the following addresses:

     https://www.ietf.org/rfc/rfc3986.txt

     https://www.rfc-editor.org/std/std66.txt

server/modules/authenticator/MySQLAuth/dbusers.c

@@ -494,7 +494,7 @@ static bool check_server_permissions(SERVICE *service, SERVER* server,
     {
         int my_errno = mysql_errno(mysql);
 
-        MXS_ERROR("[%s] Failed to connect to server '%s' (%s:%d) when"
+        MXS_ERROR("[%s] Failed to connect to server '%s' ([%s]:%d) when"
                   " checking authentication user credentials and permissions: %d %s",
                   service->name, server->unique_name, server->name, server->port,
                   my_errno, mysql_error(mysql));