core: synchronously block when logging

Previously, the logging sockets were asynchronous and if clogged we'd
lose messages. We did this to be extra careful given that PID 1 might
need to spawn the logging daemon as response to PID 1's own log messages
and we really should avoid a deadlock in that case.

As it turns out this causes loss of too many messages, hence make the
socket blocking again, however put a time limit on it to avoid unbounded
deadlocks in the unlikely case they happen.

https://bugs.freedesktop.org/show_bug.cgi?id=66664
journal: immediately sync to disk as soon as we receieve an EMERG/ALERT/CRIT message