CRM-21719 Add a check in Civi/Install/Requirements to ensure that PHP…

[seamuslee001]

… Multibyte functions are enabled on install

Overview

This follows on from the previous PR on the issue and adds the check into Civi/Install/Requirements.php as per comment from @totten

ping @eileenmcnaughton @colemanw

[totten]

(CiviCRM Review Template WORD-1.0)

• (r-jira) Pass
• (r-test) Pass
• (r-code) Pass
• (r-doc) Pass (too small; self-documenting)
• (r-maint) Pass
• (r-run) Pass: Executed by way of cv check-req. Also, manually hacked the patched code to simulate different scenarios and see they were reported.
• (r-user) Pass: Trivial impact on UX -- one of many similar checks performed during installation
• (r-tech) Pass: Trivial impact on existing integrations. Stays true to the contract of Civi/Install/Requirements.php.