Installer and upgrader improvements on older versions of PHP

Previously these two would throw PHP parse errors, instead show an error message. Also upgrade.php had 4.0.18 as minimum MySQL version, fixed that as well. Tested on PHP 5.2.17 installed on Debian 6.

Installer would throw a warning but let people install on lower PHP versions, changed it to an error and prevented them from installing. SMF would very likely break on anything lower than 5.3.8

EDIT: Fixes #1798