Bug 956841: Respect DBusWatch flags in I/O-thread watchers, r=qdot

Currently DBusWatcher registers read and write watchers on
the I/O thread unconditionally. Thus for read-only DBusWatch
structures, the write watcher on the I/O thread generates a
significant amount of CPU overhead by constantly reporting
success via calls to OnFileCanWriteWithoutBlocking.

This patch changes DBusWatcher to respect the status flags
of supplied DBusWatch structures. It will only poll for
read or write status if the DBus watch has the corresponding
flag set.

DBus keeps track of these flags and calls the toggle callback
when it needs to wait on a certain DBusWatch.