Add a setup warning for port in `mysql.host`

Summary:
A pull from GitHub recently added `mysql.port`, for explicitly configuring the MySQL port. See:

  - https://github.com/facebook/libphutil/pull/27
  - https://github.com/facebook/phabricator/pull/356

Add a setup warning for old-style configurations (which will still work properly), to get them to move to the new style.

Test Plan: {F50113}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6449