localhost/default port

When creating federated table that points to unspecified host or
localhost on unspecified port or port is 0, small memory leak occurs.

This happens because we make a copy of unix socket path, which is
never freed.

With this fix we do not make a copy of unix socket path, instead
share->socket points to MYSQL_UNIX_ADDR constant directly.

This fix is covered by a test case for BUG34788.

Affects 5.0 only.