MINOR: checks: add a flag to indicate what check is an agent

Currently to know if a check is an agent, we compare its pointer to its
servers' agent pointer. Better have a flag in its state to indicate this.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 78c4670..b02bb01 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -5146,7 +5146,7 @@
 				goto out;
 			}
 
-			newsrv->agent.state |= CHK_ST_CONFIGURED | CHK_ST_ENABLED;
+			newsrv->agent.state |= CHK_ST_CONFIGURED | CHK_ST_ENABLED | CHK_ST_AGENT;
 		}
 
 		if (!defsrv) {