Cygwin: chown: make sure ctime gets updated when necessary

Following POSIX, ensure that ctime is updated if chown succeeds,
unless the new owner is specified as (uid_t)-1 and the new group is
specified as (gid_t)-1.  Previously, ctime was unchanged whenever the
owner and group were both unchanged.

Aside from POSIX compliance, this fix makes gnulib report that chown
works on Cygwin.  This improves the efficiency of packages like GNU
tar that use gnulib's chown module.  Previously such packages would
use a gnulib replacement for chown on Cygwin.

winsup/cygwin/fhandler_disk_file.cc

@@ -887,15 +887,18 @@ fhandler_disk_file::fchown (uid_t uid, gid_t gid)
 				    aclp, MAX_ACL_ENTRIES)) < 0)
     goto out;
 
-  if (uid == ILLEGAL_UID)
-    uid = old_uid;
-  if (gid == ILLEGAL_GID)
-    gid = old_gid;
-  if (uid == old_uid && gid == old_gid)
+  /* According to POSIX, chown can be a no-op if uid is (uid_t)-1 and
+     gid is (gid_t)-1.  Otherwise, even if uid and gid are unchanged,
+     we must ensure that ctime is updated. */
+  if (uid == ILLEGAL_UID && gid == ILLEGAL_GID)
     {
       ret = 0;
       goto out;
     }
+  if (uid == ILLEGAL_UID)
+    uid = old_uid;
+  else if (gid == ILLEGAL_GID)
+    gid = old_gid;
 
   /* Windows ACLs can contain permissions for one group, while being owned by
      another user/group.  The permission bits returned above are pretty much