Run commands involving writing to journal after core is dumped

When processing core of systemd-journald, rich-core-dumper could hang if
some command attempted to write into the journal because systemd doesn't
restart journal daemon until the core of previous instance is fully read
from the kernel.

Therefore, we move the 'risky' operations after the new journald can be
launched.
When core is being omitted, dump it to /dev/null

If the crashed process is a daemon, this allows it to be restarted as
fast as possible, even before rich-core-dumper finishes.
Run logger in background

If systemd-journald just crashed, writing to journal can potentially
hang and we don't want the whole script to get stuck. Messages that
can't be written immediately will be logged after the core is dumped and
thus the daemon gets an opportunity to be restarted.
Hold a wake lock while processing a core

The lock is released when the script ends successfully or after
120 seconds.
Merge pull request #48 from xhaakon/journal-hang

Hold a wake lock while processing a core