make gsup ipa name configurable in osmo-sgsn.cfg

Add a 'gsup ipa-name' VTY command which overrides the default
IPA name used by the SGSN on the GSUP link towards the HLR.
This is required for GSUP routing in multi-SGSN networks.

The 'gsup ipa-name' option can only be set via the config file
because changing the IPA name at run-time conflicts with active
GSUP connections and routes configured in the HLR. The osmo-sgsn
program must be restarted if its IPA name needs to change.

Related: OS#3356

Change-Id: Ib2f65fed9f56b9718e8a9647e3f01dce69870c1f
diff --git a/include/osmocom/sgsn/sgsn.h b/include/osmocom/sgsn/sgsn.h
index 3a34ff9..c80355d 100644
--- a/include/osmocom/sgsn/sgsn.h
+++ b/include/osmocom/sgsn/sgsn.h
@@ -124,6 +124,12 @@
 		enum ranap_nsap_addr_enc rab_assign_addr_enc;
 	} iu;
 #endif
+
+	/* This is transmitted as IPA Serial Number tag, which is used for GSUP routing (e.g. in OsmoHLR).
+	 * This name must be set in a multi-SGSN network, and it must be unique to each SGSN.
+	 * If no name is set, the IPA Serial Number will be the same as the Unit Name,
+	 * and will be of the form 'SGSN-00-00-00-00-00-00' */
+	char *sgsn_ipa_name;
 };
 
 struct sgsn_instance {