getsubopt: work around cygwin bug

getsubopt: work around cygwin bug

On cygwin, compiling with -DGNULIB_POSIXCHECK failed because
the getsubopt link warning definition interfered with the
inclusion of the system header.  The fix, as always in these
types of problems, is to ensure that system headers are
completely included before doing any overrides.

* lib/stdlib.in.h (includes): Move unistd inclusion sooner, to
avoid conflicting with system getsubopt.
* doc/posix-functions/getsubopt.texi (getsubopt): Document the
bug.

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