core: fix priority ordering in notify-handling

core: fix priority ordering in notify-handling

Currently, we dispatch NOTIFY messages in a tight loop. Regardless how
much data is incoming, we always dispatch everything that is queued.
This, however, completely breaks priority event-handling of sd-event.
When dispatching one NOTIFY event, another completely different event
might fire, or might be queued by the NOTIFY handling. However, this
event will not get dispatched until all other further NOTIFY messages are
handled. Those might even arrive _after_ the other event fired, and as
such completely break priority ordering of sd-event (which several code
paths rely on).

Break this by never dispatching multiple messages. Just return after each
message that was read and let sd-event handle everything else.

(The patch looks scarier that it is. It basically just drops the for(;;)
 loop and re-indents the loop-content.)