Move fatal PHP functions checks to initialization

Move fatal PHP functions checks to initialization

The installer is far too late of a place to be checking if
a function does or doesn't exist if we require it for operation.
Functions can very easily (T137509) be called prior to ever
getting to the installer check page.

Also, an old installation could've gotten past the checks if
they ran update.php --skip-compat-checks. What a scary setting!

Instead, fail hard, fast and early along with PHP version checks.
All entry points are required to go through this.

While we're here, also remove the check for mbstring.func_overload
since we already would've failed at WebStart.php

Change-Id: Ib9727979af2a4741f3cd952d13a3bb350fad43e4