regcomp.c: Reorder tests to avoid throwing away work.

Prior to this patch, we did work based on a test that could be thrown
away as a result of the next test.  This reorders so that doesn't happen
perllocale: Nits
PATCH: [perl #108378] [perl #115800]

This patch solves two tickets.  Both are a result of the stringification
of a floating number being sticky, so that the character representing
the decimal point may be from an old locale.  The patch solves this by
not retaining the stringification of NVs.
Don't use locale definitions unless within scope

Prior to this patch, stringification of an NV used the current locale's
decimal point character, even outside the scope of a 'use locale'.  This
is contrary to the documentation (though one example in perllocale
omitted the 'use locale') and can lead to unexpected results.

There was one test in the core that relied on the old behavior, and
maybe more in CPAN.  This patch is being made early in 5.19 to see what
breaks.  I do believe though that any breakage is trumped by the
principal that locale rules should only be used if locales are
explicitly requested.
perllocale: Update, clarify

This rearranges and rewords portions of perllocale to be clearer,
and to add some missing caveats.