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.
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