Further refinements of ptsname_r(3) interface:

Further refinements of ptsname_r(3) interface:

 - Hide ptsname_r under __BSD_VISIBLE for now as the specification
   is not finalized at this time.
 - Keep Symbol.map sorted.
 - Avoid the interposing of ptsname_r(3) from an user application
   from breaking ptsname(3) by making the implementation a static
   method and call the static function from ptsname(3) instead.

Reported by:	kib
Reviewed by:	kib, jilles
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D26845