vty: Permit selection of other ASP protocol than M3UA

We used to have hard-coded M3UA.  Let's allow the user to configure
this per MSC using a new "asp-protocol (m3ua|sua|ipa)" VTY command.

For SUA this should just work 1:1 without any trouble.  For IPA,
this of course only changes the underlying transport without reflecting
the various differences in terms of BSSMAP ASSIGNMENT, MGCP handling,
etc.

Change-Id: I0800c709e574cedd7f5dd98be81c78782245cd13
Related: OS#2544
diff --git a/include/osmocom/bsc/bsc_msc_data.h b/include/osmocom/bsc/bsc_msc_data.h
index 345724d..c69a9ca 100644
--- a/include/osmocom/bsc/bsc_msc_data.h
+++ b/include/osmocom/bsc/bsc_msc_data.h
@@ -110,6 +110,9 @@
 		struct osmo_sccp_instance *sccp;
 		struct osmo_sccp_user *sccp_user;
 
+		/* IPA or M3UA or SUA? */
+		enum osmo_ss7_asp_protocol asp_proto;
+
 		/* Holds a copy of the our local MSC address,
 		 * this will be the sccp-address that is associated
 		 * with the A interface of this particular BSC,