fcntl-h: define O_CLOEXEC and O_EXEC if not defined; use new defines

fcntl-h: define O_CLOEXEC and O_EXEC if not defined; use new defines

* doc/posix-headers/fcntl.texi (fcntl.h): Document that
O_CLOEXEC is now defined to 0 if it is not defined, like other flags.
Also, O_EXEC is now defined to be O_RDONLY if O_EXEC is not defined.
Similarly for O_SEARCH; this last was already true, but not documented.
* lib/fcntl.in.h (O_CLOEXEC): Define to 0 if not defined.
* lib/dup-safer-flag.c (O_CLOEXEC): Remove now-useless #define.
* lib/dup3.c, lib/pipe2.c, tests/test-dup-safer.c, tests/test-fcntl.c:
Likewise.
* lib/popen-safer.c (open_noinherit): Check whether O_CLOEXEC
is zero, not whether it is defined.
* tests/test-dup3.c, tests/test-pipe2.c (main): Likewise.
* lib/progreloc.c (find_executable): Use O_EXEC rather than O_RDONLY.
* lib/open.c (open): Check for O_SEARCH as well as for O_RDONLY.