implement re-assignment to match codecs

This is the last missing piece that allows osmo-msc to make good TFO
codecs choices.

Since the codec_filter, osmo-msc properly gathers codec options and
limitations. But the MO call leg still assigns a voice channel before
getting a response from the MT call leg, and is then stuck with that.

Add the capability to adjust the MO call leg's codec in case the MT side
needs a different codec for TFO.

This is only relevant for 2G; on 3G we always have AMR/IuUP.

For inter-MSC handover, keep the behavior unchanged: offer only the
currently assigned codec to the remote side. Codec-changing HO should be
equally trivial to implement, but that is for another day.

msc_vlr_test_call's codec tests are adjusted to test the new feature in
Ib933554f826c1b4347dfa3f6c4f6fe086be8b133. For now, avoid change in
these tests by validating the first codec in SDP lists only.

Related: OS#6258
Related: osmo-ttcn3-hacks I402ed0523a2a87b83f29c5577b2c828102005d53
Change-Id: I8760feaa8598047369ef8c3ab2673013bac8ac8a
diff --git a/tests/msc_vlr/msc_vlr_test_call.c b/tests/msc_vlr/msc_vlr_test_call.c
index cb3c77b..3b91524 100644
--- a/tests/msc_vlr/msc_vlr_test_call.c
+++ b/tests/msc_vlr/msc_vlr_test_call.c
@@ -1083,6 +1083,9 @@
 			return false;
 		}
 		expect_pos++;
+
+		/* only match first codec */
+		return true;
 	}
 	if (*expect_pos) {
 		BTW("%s: %s: ERROR: mismatch: expected %s to be listed, but not found", func, desc, *expect_pos);