sd-bus: handle -EINTR return from bus_poll()

sd-bus: handle -EINTR return from bus_poll()

In sd_bus_wait(), let's convert EINTR to a return code of 0, thus asking
the caller do loop again and enter sd_bus_process() again (which will
not find any queued events). This way we'll not return an error on
something that isn't really an error. This should typically make sure
things are properly handled by the caller, magically, without eating up
the event entirely, and still giving the caller time to run some code if
they want.