mysqli: Allow for persistent database spec

Use `p:` in the database hostname specification, so `p:localhost` for
instance. This will help with SSL and remote database users.

References:
http://www.php.net/manual/en/mysqli.persistconns.php
install: Ensure no leading/trailing spaces
Merge pull request #693 from greezybacon/feature/persistent-db

mysqli: Allow for persistent database spec

Reviewed-By: Peter Rotich <peter@osticket.com>