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/mysql_auth.c

@@ -349,7 +349,7 @@ mysql_auth_authenticate(DCB *dcb)
         }
         else if (dcb->service->log_auth_warnings)
         {
-            MXS_WARNING("%s: login attempt for user '%s'@%s:%d, authentication failed.",
+            MXS_WARNING("%s: login attempt for user '%s'@[%s]:%d, authentication failed.",
                         dcb->service->name, client_data->user, dcb->remote, dcb_get_port(dcb));
 
             if (is_localhost_address(&dcb->ip) &&