Add support for if:[!]ident and ifdef:[!]ident for run/task/services

Conditional loading of stanza depending on ident is already loaded.  The
if: checks as in-band while ifdef: checks out-of-band, i.e., post eval.

The optional leading '!' negates the comparison, if NOT foo then ...

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

src/service.h

@@ -34,6 +34,7 @@ void      service_unregister     (svc_t *svc);
 void      service_runtask_clean  (void);
 void      service_reload_dynamic (void);
 void      service_update_rdeps   (void);
+void      service_mark_unavail   (void);
 
 void      service_ready_script   (svc_t *svc); /* XXX: only for pidfile plugin before notify framework */