Item12115: Make checker more emphatic

Remove the perl 5.6 checks - we only support 5.8.8+ now.

Change the UseLocale warning to an error, to make this really obvious.

git-svn-id: http://svn.foswiki.org/trunk/core@17803 0b4bb1d4-4e5a-0410-9cc4-b2b747904278