lib/locale.t: Change acceptable fail % for Win32

Commit 375f5f0648cdf36c13cb11499b332c99c710d138 fixed the Windows ctype
functions to follow POSIX rules, so there is no need to make an
exception for them.
PATCH: [perl #119425] strftime with %p

In a locale in which there is no a.m. nor p.m. indicator, a format with
only %p in it would print %p instead of nothing.
strftime: Set UTF-8 flag appropriately on return

The flag was only getting set if the format was in UTF-8.  This looks at
the return values, and sets the flag if they are non-ASCII UTF-8 and, as
a further check, if the current locale is a UTF-8 one.
perlapi: Nit, and mention NUL for some functions

This adds the mention of embedded or terminating NULs for some functions
where it wasn't previously indicated.
POSIX.xs: Add a const to a parameter
locale.c: Fix uncomplemented 'if' test

Somehow the ! in this if () got dropped, and there were no tests to
catch it.  Now both are remedied.