forkpty, openpty: prefer glibc's const-safe prototype

forkpty, openpty: prefer glibc's const-safe prototype

This silences a compiler warning for test-forkpty.c.

* lib/forkpty.c (rpl_forkpty): New file.
* lib/openpty.c (rpl_openpty): Likewise.
* modules/forkpty (Files): Distribute it.
* modules/openpty (Files): Likewise.
* m4/pty_h.m4 (gl_PTY_H_DEFAULTS): Add new witnesses.  Move decl
check...
* m4/pty.m4 (gl_FORKPTY, gl_OPENPTY): ...here.  Request
replacement for for non-const BSD signature.
* modules/pty (Makefile.am): Substitute witnesses.
* lib/pty.in.h (forkpty, openpty): Declare replacements.
* tests/test-forkpty.c: Update signature check.
* tests/test-openpty.c: Likewise.
* doc/glibc-functions/forkpty.texi (forkpty): Document the fix.
* doc/glibc-functions/openpty.texi (openpty): Likewise.
Reported by Bruno Haible.

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