openat: fix openat bugs on Solaris 9

openat: fix openat bugs on Solaris 9

openat(fd,"file/",O_RDONLY) mistakenly succeeded.

* lib/openat.c (rpl_openat): Work around Solaris 9 bug.
* m4/openat.m4 (gl_FUNC_OPENAT): Also replace openat on Solaris.
* modules/openat (Depends-on): Add open.
* m4/fcntl_h.m4 (gl_FCNTL_H_DEFAULTS): Provide new default.
* modules/fcntl-h (Makefile.am): Substitute it.
* lib/fcntl.in.h (openat): Declare replacement.
* doc/posix-functions/openat.texi (openat): Document this.

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