window: fix meta_window_is_remote across hostname changes

meta_window_is_remote compares a cached copy of the system hostname
with the hostname of the client window
(as presented by the WM_CLIENT_MACHINE property).

Of course, the system hostname can change at any time, so caching
it is wrong. Also, the WM_CLIENT_MACHINE property won't necessarily
change when the system hostname changes, so comparing it with the
new system hostname is wrong, too.

This commit makes the code call gethostname() at the time
WM_CLIENT_MACHINE is set, check whether it's remote then, and cache
that value, rather than comparing potentially out of sync hostnames
later.

https://bugzilla.gnome.org/show_bug.cgi?id=688716