chmod issue on 3.1.7.

["Kaz Kylheku (Cygwin)" <743-406-3965@kylheku.com>]

Hi all,

Running this Cygwin on a Windows 10 system:

   0:DESKTOP-K8055OB:~$ uname -a
   CYGWIN_NT-10.0-WOW DESKTOP-K8055OB 3.1.7(0.340/5/3) 2020-08-22 19:03 
i686 Cygwi

When a file is created, and permissions set as follows:

   0:DESKTOP-K8055OB:~$ touch tempfile
   0:DESKTOP-K8055OB:~$ chmod 03777 tempfile
   0:DESKTOP-K8055OB:~$ ls -l tempfile
   -rwsrwsrwt 1 kaz kaz 0 Oct 10 08:59 tempfile

Then "chmod u=" is not able to clear the owner's permissions to nothing:

   0:DESKTOP-K8055OB:~$ chmod u= tempfile
   0:DESKTOP-K8055OB:~$ ls -l tempfile
   -rwxrwsrwt 1 kaz kaz 0 Oct 10 08:59 tempfile

As you can see, it has no effect. The expected value is ----rwsrwt.

I tried both with 64 and 32 bit Cygwin: same deal.

This is not a problem with the chmod utility.  I ran into this as a 
failing
test case against a chmod library function in a programming language.

http://www.kylheku.com/cgit/txr/tree/tests/018/chmod.tl

The test cases pass until the "u=", which fails in the same way.
This does not use the chmod utility.

It's an issue with the chmod system call.

This used to work on my older Cygwin installation, which was around 2.5.