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_ussd.c b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
index 80d6e77..f6dbef2 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_ussd.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
@@ -68,7 +68,7 @@
 
 static int forward_sccp(struct bsc_nat *nat, struct msgb *msg)
 {
-	struct sccp_connections *con;
+	struct nat_sccp_connection *con;
 	struct bsc_nat_parsed *parsed;
 
 
@@ -255,7 +255,7 @@
 			 ntohl(addr.s_addr), 5001, 0, ussd_listen_cb, nat);
 }
 
-static int forward_ussd_simple(struct sccp_connections *con, struct msgb *input)
+static int forward_ussd_simple(struct nat_sccp_connection *con, struct msgb *input)
 {
 	struct msgb *copy;
 	struct bsc_nat_ussd_con *ussd;
@@ -279,7 +279,7 @@
 	return 0;
 }
 
-static int forward_ussd(struct sccp_connections *con, const struct ussd_request *req,
+static int forward_ussd(struct nat_sccp_connection *con, const struct ussd_request *req,
 			struct msgb *input)
 {
 	struct msgb *msg, *copy;
@@ -323,7 +323,7 @@
 	return 0;
 }
 
-int bsc_check_ussd(struct sccp_connections *con, struct bsc_nat_parsed *parsed,
+int bsc_check_ussd(struct nat_sccp_connection *con, struct bsc_nat_parsed *parsed,
 		   struct msgb *msg)
 {
 	uint32_t len;