machined: some modernizations

machined: some modernizations

A couple of minor modernizations:

1. Don't unnecessarily export functions we don't call outside of
   machined.c

2. Use cleanup logic for the manager object.

3. Propagate errors properly from manager_new(). So far if
   sd_event_new() returns EMFILE/ENFILE for some reason we would have
   logged that as log_oom(), which isn#t right, really.

4. Handle SIGTERM/SIGINT cleanly. It's easy, and prettier then letting
   the kernel just abort us. It also makes it possible to valgrind
   machined properly.