log: Add support for journal logging

This logging is available if the software was build with the configure
option --with-systemd-journal.

It will be enabled by default if libsystemd is found.

The runtime parameter --log-journal enables to output of log messages
to the systemd journal.

Please note that the journal priority field has the same value as the
syslog level so no conversion is required here.

src/main.c

@@ -141,6 +141,7 @@ main (int argc, char *argv[])
 
     if (!mm_log_setup (mm_context_get_log_level (),
                        mm_context_get_log_file (),
+                       mm_context_get_log_journal (),
                        mm_context_get_log_timestamps (),
                        mm_context_get_log_relative_timestamps (),
                        &err)) {