log: Add support for the systemd journal

The journal is a component of systemd, that captures Syslog messages,
Kernel log messages, initial RAM disk and early boot messages as well
as messages written to STDOUT/STDERR of all services, indexes them and
makes this available to the user.

It can be used in parallel, or in place of a traditional syslog daemon,
such as rsyslog or syslog-ng.

The journal offers a couple of improvements over traditional logging
facilities (e.g. advanced filtering capabilities).

This patch adds support for logging directly to the journal using its
native API.

src/daemon/main.c

@@ -817,7 +817,11 @@ int main(int argc, char *argv[]) {
 #endif
 
         if (!conf->log_target) {
+#ifdef HAVE_JOURNAL
+            pa_log_target target = { .type = PA_LOG_JOURNAL, .file = NULL };
+#else
             pa_log_target target = { .type = PA_LOG_SYSLOG, .file = NULL };
+#endif
             pa_log_set_target(&target);
         }