Add WIP code for SCCP User Adaptation (SUA) and SCCP User SAP

The idea of this code is to
* provide a SCCP User SAP as boundary between the User of SCCP
  or SCCP-like transport like SUA
* implement the minimum subset of SUA to transport RANAP messages
  betweene HNB-GW and MSC/SGSN

At this point
* we don't yet implement the proper state machines and timer
* we don't imp[lement the SCCP RESET procedure
* we don't implement AS/ASP management

The code is full of FIXMEs whihc hopefully will get fixed gradually.

After some cleanup + verification, it should move to a library, possibly
either replacing/renaming libomo-sccp, or adding it to libosmo-netif?
diff --git a/sigtran/sccp_sap.c b/sigtran/sccp_sap.c
new file mode 100644
index 0000000..207d7ea
--- /dev/null
+++ b/sigtran/sccp_sap.c
@@ -0,0 +1,43 @@
+#include <string.h>
+#include <osmocom/core/utils.h>
+
+#include "sccp_sap.h"
+
+const struct value_string osmo_scu_prim_names[] = {
+	{ OSMO_SCU_PRIM_N_CONNECT,		"N-CONNECT" },
+	{ OSMO_SCU_PRIM_N_DATA,			"N-DATA" },
+	{ OSMO_SCU_PRIM_N_EXPEDITED_DATA,	"N-EXPEDITED-DATA" },
+	{ OSMO_SCU_PRIM_N_DISCONNECT,		"N-DISCONNECT" },
+	{ OSMO_SCU_PRIM_N_RESET,		"N-RESET" },
+	{ OSMO_SCU_PRIM_N_INFORM,		"N-INFORM" },
+	{ OSMO_SCU_PRIM_N_UNITDATA,		"N-UNITDATA" },
+	{ OSMO_SCU_PRIM_N_NOTICE,		"N-NOTICE" },
+	/* management */
+	{ OSMO_SCU_PRIM_N_COORD,		"N-COORD" },
+	{ OSMO_SCU_PRIM_N_STATE,		"N-STATE" },
+	{ OSMO_SCU_PRIM_N_PCSTATE,		"N-PCSATE" },
+	{ 0, NULL }
+};
+
+const struct value_string osmo_prim_op_names[] = {
+	{ PRIM_OP_REQUEST,			"request" },
+	{ PRIM_OP_RESPONSE,			"response" },
+	{ PRIM_OP_INDICATION,			"indication" },
+	{ PRIM_OP_CONFIRM,			"confirm" },
+	{ 0, NULL }
+};
+
+static char prim_name_buf[128];
+
+char *osmo_sccp_prim_name(struct osmo_prim_hdr *oph)
+{
+	const char *name = get_value_string(osmo_scu_prim_names, oph->primitive);
+
+	prim_name_buf[0] = '\0';
+	strncpy(prim_name_buf, name, sizeof(prim_name_buf)-1);
+	prim_name_buf[sizeof(prim_name_buf)-1] = '\0';
+	name = get_value_string(osmo_prim_op_names, oph->operation);
+	strncat(prim_name_buf, name, sizeof(prim_name_buf)-strlen(prim_name_buf)-2);
+
+	return prim_name_buf;
+}