Another chmod issue on 3.1.7, Win 10.

[Kaz Kylheku]

Initial conditions:

   0:BLACKBOX:~/txr$ rm testfile
   0:BLACKBOX:~/txr$ touch testfile
   0:BLACKBOX:~/txr$ chmod = testfile
   0:BLACKBOX:~/txr$ ls -l testfile
   ---------- 1 kaz kaz 0 Dec 24 13:23 testfile

Now, light up all the bits, like a Christmas tree---appropriate
for December 24:

   0:BLACKBOX:~/txr$ chmod ugo+rwxs testfile
   0:BLACKBOX:~/txr$ ls -l testfile
   -rwsrwsrwx 1 kaz kaz 0 Dec 24 13:23 testfile

Well, almost all the bits: I didn't add "t", but it makes
no difference. Anyway, so far, so good. Now, punch a hole in
the group permissions:

   0:BLACKBOX:~/txr$ ls -l testfile
   -rws---rwx+ 1 kaz kaz 0 Dec 24 13:23 testfile

No idea what that + is hinting at, which indicates a problem.
But the group perms did get masked out. Let's keep going to the
main problem: repeat the second step to turn on all the bits:

   0:BLACKBOX:~/txr$ chmod ugo+rwxs testfile
   0:BLACKBOX:~/txr$ ls -l testfile
   -rws--Srwx+ 1 kaz kaz 0 Dec 24 13:23 testfile

What??? The expected perms are "-rwsrwsrwx". It did not set the group 
read
or write at all, and that funny  + is still hanging around.

This test case worked on Cygwin 2.5 on Win 7.