Latch LC_NUMERIC during critical sections

Latch LC_NUMERIC during critical sections

It is possible for operations on threaded perls which don't 'use locale'
to still change the locale.  This happens when calling
POSIX::localeconv() and I18N::Langinfo(), and in earlier perls, it can
happen for other operations when perl has been initialized with the
environment causing the various locale categories to not have a uniform
locale.

This commit causes the areas where the locale for this category should
predictably be in one or the other state to be a critical section where
another thread can't interrupt and change it.  This is a separate
mutex, so that only these particular operations will be held up.