posix_fadvise() *returns* error codes but does not set errno

Also updates the fhandler_*::fadvise implementations to adhere to the same
semantics.

winsup/cygwin/syscalls.cc

@@ -2937,7 +2937,7 @@ posix_fadvise (int fd, off_t offset, off_t len, int advice)
   if (cfd >= 0)
     res = cfd->fadvise (offset, len, advice);
   else
-    set_errno (EBADF);
+    res = EBADF;
   syscall_printf ("%R = posix_fadvice(%d, %D, %D, %d)",
 		  res, fd, offset, len, advice);
   return res;