perllocale.pod: Fix misstatements, and some nits

"$!" (stringified) is always subject to the current locale, even outside
'use locale'.  The motiviation, I believe, for this is that these are
operating system messages that are generally of most value to the
end-user, and so should be expressed in terms that are understandable to
the end-user, regardless of what the Perl programmer did or didn't do.