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.h

@@ -115,6 +115,7 @@ extern char  *sdown;
 extern char  *network;
 extern char  *hostname;
 extern char  *runparts;
+extern char  *osheading;
 extern uev_ctx_t *ctx;
 
 #endif /* FINIT_H_ */