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