add sdp_msg API: SDP parsing/composition

Rationale: in order to add full SDP to the MNCC protocol (upcoming patch
I8c3b2de53ffae4ec3a66b9dabf308c290a2c999f), we need to parse and compose SDP
messages. Obviously, libosmo-mgcp-client already contains similar code, but
that is unfortunately heavily glued to the actual MGCP implementation. The
simplest solution is to create this separate implementation, copy-pasting from
the existing libosmo-mgcp-client code as is convenient.

This API is added here to probe whether it works well. When it does, the
intention is to "move it up" to osmo-mgw and overhaul the SDP parsing in our
MGCP client and MGCP server APIs using this same API.

Change-Id: If3ce23cd5bab15e2ab4c52ef3e4c75979dffe931
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 27c1205..864ac7c 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -2,6 +2,7 @@
 	sms_queue \
 	msc_vlr \
 	db_sms \
+	sdp_msg \
 	$(NULL)
 
 if BUILD_SMPP