MINOR: checks: use check->state instead of srv->state & SRV_CHECKED

Having the check state partially stored in the server doesn't help.
Some functions such as srv_getinter() rely on the server being checked
to decide what check frequency to use, instead of relying on the check
being configured. So let's get rid of SRV_CHECKED and SRV_AGENT_CHECKED
and only use the check's states instead.
diff --git a/src/server.c b/src/server.c
index a316daa..d91c726 100644
--- a/src/server.c
+++ b/src/server.c
@@ -34,7 +34,7 @@
 {
 	const struct server *s = check->server;
 
-	if ((s->state & SRV_CHECKED) && (check->health == check->rise + check->fall - 1))
+	if ((check->state & CHK_ST_CONFIGURED) && (check->health == check->rise + check->fall - 1))
 		return check->inter;
 
 	if (!(s->state & SRV_RUNNING) && check->health == 0)