ptsname.3: accommodate upcoming POSIX Issue 8 ptsname_r

POSIX has accepted a proposal[1] to add glibc-compatible ptsname_r. It
indicates an error by returning the error number, rather than returning
-1 and setting errno. Update RETURN VALUES in ptsname_r's man page now
to encourage folks to test that the return value != 0 rather than == -1.

[1] https://www.austingroupbugs.net/bug_view_page.php?bug_id=508

Reported by: Collin Funk
Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42204