connection: Make wl_closure_destroy() close fds of undispatched closures

When we have a closure that can't be dispatched for some reason, and it
contains file descriptors, we must close those descriptors to prevent
leaking them.

Previous commits ensure that only FDs belonging to this invocation of
the closure, i.e. not FDs provided by the client for marshalling, nor
FDs which have already been dispatched to either client or server, will
be left in the closure by destroy time.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>