locale.c: Generalize stdize_locale()

locale.c: Generalize stdize_locale()

This function is rewritten to handle LC_ALL, and to make it easier to
add new checks.

There is also a change, which I think is an improvement, in that everything
starting with a \n is trimmed, instead of just a trailing \n.

A couple of calls to stdize_locale() are removed, as they are redundant,
because they are called only as a result of Perl_setlocale() being
called, and that ends up calling stdize_locale always, early on.

The call to savepv() is also moved in a couple cases to after the result
is known to not be NULL

I originally had such a new check in mind, but it turned out that doing
it here didn't solve the problem, so this commit has been amended
(before ever being pushed) to not include that.
chomped.