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/mm-context.c

@@ -89,6 +89,7 @@ mm_context_get_no_auto_scan (void)
 
 static const gchar *log_level;
 static const gchar *log_file;
+static gboolean     log_journal;
 static gboolean     log_show_ts;
 static gboolean     log_rel_ts;
 
@@ -103,6 +104,13 @@ static const GOptionEntry log_entries[] = {
         "Path to log file",
         "[PATH]"
     },
+#if WITH_SYSTEMD_JOURNAL
+    {
+        "log-journal", 0, 0, G_OPTION_ARG_NONE, &log_journal,
+        "Log to systemd journal",
+        NULL
+    },
+#endif
     {
         "log-timestamps", 0, 0, G_OPTION_ARG_NONE, &log_show_ts,
         "Show timestamps in log output",
@@ -142,6 +150,12 @@ mm_context_get_log_file (void)
     return log_file;
 }
 
+gboolean
+mm_context_get_log_journal (void)
+{
+    return log_journal;
+}
+
 gboolean
 mm_context_get_log_timestamps (void)
 {