log-control-api: add generic D-Bus interface for querying/setting log level/target

log-control-api: add generic D-Bus interface for querying/setting log level/target

Let's define a new, generic bus interface that any daemon can implement
for querying/setting the log level.

We can turn this into something more powerful later on, but for now,
only expose three properties: the log level, log target and the syslog
identifier (with the former two being writable).

This is supposed to be generic, so that it can be implemented by 3rd
party daemons too, eventually.