nat: Rename "struct sccp_connections" to "struct nat_sccp_connection"

The name sccp_connection is used in the osmo-sccp code, sccp_connections
was used in the NAT for tracking a sccp_connection. Rename it so it is
obvious that the struct belongs to the nat.

The rename was done with sed:
$ sed -i s,"struct sccp_connections","struct nat_sccp_connection",g \
		include/openbsc/*.h src/osmo-bsc_nat/* tests/*/*
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_filter.c b/openbsc/src/osmo-bsc_nat/bsc_nat_filter.c
index 5ab3a97..8ccc262 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_filter.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_filter.c
@@ -290,7 +290,7 @@
 
 static int _dt_check_id_resp(struct bsc_connection *bsc,
 			     uint8_t *data, unsigned int length,
-			     struct sccp_connections *con,
+			     struct nat_sccp_connection *con,
 			     struct bsc_nat_reject_cause *cause)
 {
 	char mi_string[GSM48_MI_SIZE];
@@ -404,7 +404,7 @@
 }
 
 int bsc_nat_filter_dt(struct bsc_connection *bsc, struct msgb *msg,
-		struct sccp_connections *con, struct bsc_nat_parsed *parsed,
+		struct nat_sccp_connection *con, struct bsc_nat_parsed *parsed,
 		struct bsc_nat_reject_cause *cause)
 {
 	uint32_t len;