MTR is broken on systems with IPv6 disabled

Some systems report IPv6 as supported (as in, getaddrinfo() returns both AF_INET6 and AF_INET addrinfo's), but an attempt to create an AF_INET6 socket fails, because IPv6 support is either not fully available or disabled.

This does not prevent the server from starting up, because activate_tcp_port() falls back to AF_INET if creating an AF_INET6 socket fails. However, the "Failed to create a socket for IPv6" error is printed in the error log, which makes all MTR tests on such systems fail with the following symptoms:

main.connect                             [ fail ]  Found warnings/errors in server log file!

        Test ended at 2015-04-30 08:39:03

line

150430  8:38:52 [ERROR] Failed to create a socket for IPv6 '::': errno: 97.

150430  8:38:52 [ERROR] Failed to create a socket for IPv6 '::': errno: 97.

^ Found warnings in /home/ubuntu/install/mariadb-10.1.5/usr/local/mysql/mysql-test/var/log/mysqld.1.err

This is a regression introduced by the patch for MDEV-4379, as before that change active_tcp_port() always tried creating an AF_INET socket before AF_INET6.

A similar problem has been fixed in MySQL with https://github.com/mysql/mysql-server/commit/da66e3850d0b186a3c86e06daa4bc3208ad5cc62