Add support for auto-detetcing OS heading for progress

This patch removes the cognitive overhead of having to manually set your
OS heading, --with-heading="Foo OS vX.YY".  As of this patch, Finit by
default extracts PRETTY_NAME from /etc/os-release.  It is now possible
to also disable the heading entirely using --without-heading

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/finit.c

@@ -66,6 +66,7 @@ char *network   = NULL;
 char *hostname  = NULL;
 char *rcsd      = FINIT_RCSD;
 char *runparts  = NULL;
+char *osheading = NULL;
 
 uev_ctx_t *ctx  = NULL;		/* Main loop context */
 svc_t *wdog     = NULL;		/* No watchdog by default */
@@ -90,7 +91,14 @@ static void banner(void)
 	 */
 	plugin_run_hooks(HOOK_BANNER);
 
-	print_banner(INIT_HEADING);
+#ifdef INIT_OSHEADING
+	osheading = INIT_OSHEADING;
+	if (osheading) {
+		if (!osheading[0])
+			osheading = release_heading();
+		print_banner(osheading);
+	}
+#endif
 }
 
 /*