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/backend.c b/src/backend.c
index 55debef..c680777 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1443,7 +1443,7 @@
 	smp->flags = SMP_F_VOL_TEST;
 	smp->type = SMP_T_BOOL;
 	if (!(srv->state & SRV_MAINTAIN) &&
-	    (!(srv->state & SRV_CHECKED) || (srv->state & SRV_RUNNING)))
+	    (!(srv->check.state & CHK_ST_CONFIGURED) || (srv->state & SRV_RUNNING)))
 		smp->data.uint = 1;
 	else
 		smp->data.uint = 0;