cosmetic: rename db_subscr_get() to db_subscr_get_by_imsi()

There will be more additions, _by_msisdn() and _by_id(), to serve the upcoming
VTY commands, to allow flexibly selecting subscribers as in the old OsmoNITB.

Change-Id: I32fa676ccc5c10eba834c4390c8a42476b9c1961
diff --git a/src/ctrl.c b/src/ctrl.c
index 81de961..b034cd7 100644
--- a/src/ctrl.c
+++ b/src/ctrl.c
@@ -38,7 +38,7 @@
 	struct lu_operation *luop = NULL;
 	struct osmo_gsup_conn *co;
 
-	if (db_subscr_get(ctx->dbc, cmd->value, NULL) < 0) {
+	if (db_subscr_get_by_imsi(ctx->dbc, cmd->value, NULL) < 0) {
 		cmd->reply = "Subscriber Unknown in HLR";
 		return CTRL_CMD_ERROR;
 	}