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/pulsecore/log.h

@@ -35,6 +35,9 @@
 typedef enum pa_log_target_type {
     PA_LOG_STDERR,      /* default */
     PA_LOG_SYSLOG,
+#ifdef HAVE_JOURNAL
+    PA_LOG_JOURNAL,     /* systemd journal */
+#endif
     PA_LOG_NULL,        /* to /dev/null */
     PA_LOG_FILE,        /* to a user specified file */
     PA_LOG_NEWFILE,     /* with an automatic suffix to avoid overwriting anything */