Cygwin: chown: make sure ctime gets updated when necessary

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.