unistd: guarantee STDIN_FILENO here, for OS/2 EMX

unistd: guarantee STDIN_FILENO here, for OS/2 EMX

* lib/unistd.in.h (STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO):
Guarantee a definition.
* doc/posix-headers/unistd.texi (unistd.h): Document the bug.
* modules/unistd-safer (Depends-on): Add dependency on unistd.
* lib/c-stack.c (STDERR_FILENO): Rely on <unistd.h>.
* lib/dup-safer.c (STDERR_FILENO): Likewise.
* lib/execute.c (STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO):
Likewise.
* lib/fd-safer.c (STDIN_FILENO, STDERR_FILENO): Likewise.
* lib/fopen-safer.c (STDERR_FILENO): Likewise.
* lib/pipe.c (STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO):
Likewise.
* lib/tmpfile-safer.c (STDERR_FILENO): Likewise.
* tests/test-posix_spawn1.c (STDIN_FILENO, STDOUT_FILENO)
(STDERR_FILENO): Likewise.
* tests/test-posix_spawn2.c (STDIN_FILENO, STDOUT_FILENO)
(STDERR_FILENO): Likewise.
* tests/test-posix_spawn3.c (STDIN_FILENO, STDOUT_FILENO)
(STDERR_FILENO): Likewise.
Reported by Elbert Pol.

Signed-off-by: Eric Blake <ebb9@byu.net>