Fix path ownership restoration problems on Cygwin.

Fix path ownership restoration problems on Cygwin.

It turns out that Cygwin won't allow you to chown() a path to an
unknown uid or gid, even when "root".

For now, make that a deferred error on Cygwin, rework the tests to
avoid it when possible, and disable the tests (on Cygwin) that require
it.

For the record, it appears that tar doesn't normally hit this problem
on Cygwin because it uses "geteuid() == 0" to detect super-user
status, which won't be true in the normal case, even if the user is an
administrator.

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