[PATCHv3]: Logging cleanup and systemd journal support

[Torsten Hilbrich]

This series of patches is based on master.

It start with cleaning up some parts of the logging code. The the
existing logging support (supporting syslog and file-based
logging) is split into two different functions using a function
pointer to select them. Then the support for logging using the
systemd journal is added.

v2:

The feedback provided by Aleksander Morgado is now part of the
patches. I documented the use of the syslog loglevel in patch 4
as we need to be able to convert both the mm loglevel and the
glib loglevel to the parameter of the log_backend function.
As both syslog and journal output uses the same priority value
using syslog level is preferable as it better supports the wider
range coming from GLogLevelFlags.

v3:

Rebase to master (0e854644dbd896577eaa8f9b05c10aa6283b9533)

[PATCH 1/5] log: Refactor evaluation of log level
[PATCH 2/5] log: Refactor log level text output
[PATCH 3/5] log: Add mapping from glib loglevel to syslog priority
[PATCH 4/5] log: Introduce function pointer to handle different log
[PATCH 5/5] log: Add support for journal logging