is_superuser: test for group 544 or 0 on cygwin

is_superuser: test for group 544 or 0 on cygwin

This appears to be the appropriate way to check for admin status in
cygwin right now: https://cygwin.com/ml/cygwin/2015-02/msg00057.html

Thanks to at least Andrew Skretvedt, Ruvim Pinka, renpj, and Iar De
for reporting the problem, Ruvim Pinka, Paul Kronenwetter, and renpj
for proposing earlier solutions, and Ben Kelly and Johannes Berg for
helping test this approach.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>