_dbus_loop_iterate: if the kernel says a fd is bad, stop watching it

_dbus_loop_iterate: if the kernel says a fd is bad, stop watching it

Again, this shouldn't happen - modules are responsible for cleaning up
their watches - but the failure mode here is really bad: if we leave an
invalid fd in the set, every poll() call will instantly return, marking
it as POLLNVAL. The result is that dbus-daemon busy-loops on poll()
without responding to I/O, so the bad watch will probably never be
cleared up.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=32992
Bug-NB: NB#200248
Reviewed-by: Colin Walters <walters@verbum.org>