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/testsuite.at b/tests/testsuite.at
index c0788b9..946d0db 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -107,3 +107,10 @@
 cat $abs_srcdir/msc_vlr/msc_vlr_test_ss.err > experr
 AT_CHECK([$abs_top_builddir/tests/msc_vlr/msc_vlr_test_ss], [], [expout], [experr])
 AT_CLEANUP
+
+AT_SETUP([sdp_msg_test])
+AT_KEYWORDS([sdp_msg_test])
+cat $abs_srcdir/sdp_msg/sdp_msg_test.ok > expout
+cat $abs_srcdir/sdp_msg/sdp_msg_test.err > experr
+AT_CHECK([$abs_top_builddir/tests/sdp_msg/sdp_msg_test], [], [expout], [experr])
+AT_CLEANUP