Support non-standard port numbers for MySQLi

Validate port during installation

Add help information about the non-standard port number to the popup tips in
the install screen.

Support MySQL 'localhost' connections using a non-standard port number.
Ordinarily, MySQL will ignore the port setting on *nix systems if the host
is specified as 'localhost'

Implements/Fixes #769
Merge pull request #775 from greezybacon/feature/mysqli-nonstandard-port

Support non-standard port numbers for MySQLi

Reviewed-By: Peter Rotich <peter@osticket.com>