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/conf.c

@@ -1011,6 +1011,9 @@ int conf_reload(void)
 	/* Mark any reverse deps as chenaged. */
 	service_update_rdeps();
 
+	/* Prune according to ifdef:[!]ident */
+	service_mark_unavail();
+
 	/* Set up top-level cgroups */
 	cgroup_config();
 done: