Logging over D-Bus
Historically, mms-engine logging was done by either running it from the console or by launching it programmatically and pulling the log out of it's stdout. That requires restarting mms-enging and doesn't allow attaching to an already running process. And doesn't play nice with multi-user setup.
Switch mms-engine to libdbuslog-style logging, like connman, ofono and nfcd.
A few unrelated changes are included too, as separate commits.