ptsname_r: new module

ptsname_r: new module

For now, this replacement focuses solely on compilation
compatibility, and assumes that isatty() and ttyname_r() work
on a master side pty; if this assumption fails, or if
thread-safety is also required, then a later patch can follow
the lead of strerror_r.c in wrapping the system ptsname()
with a lock.

* modules/ptsname_r: New module.
* m4/ptsname_r.m4 (gl_FUNC_PTSNAME_R): New file.
* lib/ptsname.c (__ptsname_r): Split...
* lib/ptsname_r.c: ...into new file.
* m4/stdlib_h.m4 (gl_STDLIB_H): Check for decl.
(gl_STDLIB_H_DEFAULTS): Set witness defaults.
* modules/stdlib (Makefile.am): Substitute witnesses.
* lib/stdlib.in.h (ptsname_r): Declare it.
* doc/glibc-functions/ptsname_r.texi (ptsname_r): Document it.
* MODULES.html.sh (Misc): Likewise.
* modules/ptsname (Depends-on): Alter dependency.
* doc/posix-functions/ptsname.texi (ptsname): Mention new module.

Signed-off-by: Eric Blake <eblake@redhat.com>