Update xdmod-check-config script

[jtpalmer]

Description

Adds checks for PHP APCu and Multibyte String functions to make sure the corresponding extensions are installed and enabled.

Motivation and Context

When I was removing the mcrypt requirement I noticed that checks for these extensions were missing. These checks serve as a sanity check to assist in debugging.

Tests performed

Tested manually.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jtpalmer]

The test for mbstring isn't strictly necessary since we're including a polyfill (symfony/polyfill-mbstring is a dependency of a dependency of Silex). That brings up the question of why we include php-mbstring as a dependency of the RPM package if we're going to ship a polyfill.