db,sms: Rename db_sms_mark_sent() to db_sms_mark_delivered()

In MT-SMS the message is being delivered. Make the naming follow
that. The schema still refers to "sent" while it should be "delivered"
too.
diff --git a/openbsc/src/libmsc/gsm_04_11.c b/openbsc/src/libmsc/gsm_04_11.c
index 975a263..58b68ed 100644
--- a/openbsc/src/libmsc/gsm_04_11.c
+++ b/openbsc/src/libmsc/gsm_04_11.c
@@ -554,7 +554,7 @@
 	}
 
 	/* mark this SMS as sent in database */
-	db_sms_mark_sent(sms);
+	db_sms_mark_delivered(sms);
 
 	send_signal(S_SMS_DELIVERED, trans, sms, 0);