forkpty, openpty: split functions into new modules

forkpty, openpty: split functions into new modules

The pty module was doing too much - replacing the header and checking
for functions.  In the future, when we provide a working forkpty for
Solaris and HP-UX, and a stub for mingw, then the separation will be
even more useful.

* modules/pty (Makefile.am): Substitute new witnesses.
(Libraries): Move library detection...
* modules/forkpty: ...into new module.
* modules/openpty: Another new module.
* modules/pty-tests: Rename and split...
* modules/forkpty-tests: ...to this...
* modules/openpty-tests: ...and this.
* tests/test-pty.c: Rename and split...
* tests/test-forkpty.c: ...to this...
* tests/test-openpty.c: ...and this.
* m4/pty_h.m4 (gl_PTY_H_DEFAULTS): Add new witnesses.
(gl_PTY): Split library searching...
* m4/pty.m4 (gl_PTY_LIB): ...into new file.
(gl_FORKPTY, gl_OPENPTY): New macros.
* lib/pty.in.h (forkpty, openpty): Honor new witnesses.
* NEWS: Mention the split.
* MODULES.html.sh (Misc): Document the modules.
* doc/glibc-functions/forkpty.texi (forkpty): Likewise.
* doc/glibc-functions/openpty.texi (openpty): Likewise.

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