libc: setlocale must reject unavailable locales and return a name newlocale accepts

[gburd]

Problem

OSv has no on-disk locale database and provides only the C locale, but
setlocale() returned "C.UTF-8" unconditionally and ignored its arguments.
That is observably wrong for a portable application in two ways:

1. A request for a locale OSv cannot provide (for example "en_US.UTF-8")
   must return NULL so the caller learns it is unavailable. The stub
   silently reported success for a locale that was never actually in effect.

2. setlocale() must return a name that newlocale() will accept, because
   portable software round-trips the two. PostgreSQL derives a new database's
   default collation from setlocale() and feeds that name back into
   newlocale(). The stub returned "C.UTF-8", which OSv's newlocale()
   rejects, so a cluster initialized with LC_COLLATE=C nonetheless stamped
   every CREATE DATABASE with datcollate="C.UTF-8" and then could not open
   it (could not create locale "C.UTF-8": ENOENT). Any application that
   creates its own database at runtime was blocked; clusters whose locale was
   baked at initdb time happened to avoid it, which hid the bug.

Fix

Replace the stub with a small correct implementation: report the C locale
(which newlocale() accepts) and accept only the C-family names OSv can
actually provide ("", C, POSIX, C.UTF-8), failing any other request.
Character-set encoding (for example UTF8 in PostgreSQL) is independent of the
libc locale and is unaffected.

Testing

Built and booted; a stock PostgreSQL cluster on OSv can now CREATE DATABASE
and open the result, which previously failed with the ENOENT above.