fdatasync: port to MacOS X 10.7

fdatasync: port to MacOS X 10.7

MacOS X 10.7 added an fdatasync syscall and exports a symbol for it,
but fails to declare it anywhere.  Lacking any hard evidence that this
syscall is broken, we will just provide the declaration and use it.

* m4/fdatasync.m4 (gl_FUNC_FDATASYNC): Check for present but not
declared.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Another default.
* modules/unistd (Makefile.am): Substitute it.
* lib/unistd.in.h (fdatasync): Declare on MacOS.
* doc/posix-functions/fdatasync.texi (fdatasync): Document it.

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