Fix Perl_langinfo() non-threaded bug

Fix Perl_langinfo() non-threaded bug

Perl_langinfo() is supposed to return a pointer to internal storage that
is supposed to remain valid until the next call to it.  That should come
automatically on single-threaded perls.  The previous version took
advantage of this to avoid copying the result to a buffer, and just
called plain nl_langinfo().  However, it turns out that some systems
destroy the internal space also when a setlocale() is done.  That means
the result must be copied in all instances.