posix_openpt: new module

posix_openpt: new module

* modules/posix_openpt: New module.
* m4/posix_openpt.m4: New file.
* lib/posix_openpt.c: Likewise.
* m4/stdlib_h.m4 (gl_STDLIB_H): Check for decl.
(gl_STDLIB_H_DEFAULTS): Set defaults.
* modules/stdlib (Makefile.am): Substitute macros.
* lib/stdlib.in.h (posix_openpt): Declare.
* MODULES.html.sh (systems lacking POSIX:2008): Document it.
* doc/posix-functions/posix_openpt.texi (posix_openpt): Likewise.
* modules/posix_openpt-tests: New test module.
* tests/test-posix_openpt.c: New test.