drop code dup between libosmo-mgcp-client and libosmo-mgcp

Remove mgcp_common.c and replace with mgcp_common.h.

Move mgcp_common.h from mgcp_client/ to mgcp/;
Place a compile-time copy of it back in mgcp_client/.

Add builddir/include to compiler -I paths to find generated header.

Rationale:
- Keep separate copies of the file for each of the library to not require
  debian dependencies between the two libraries.
- Avoid code dup by copying during 'make' (think: a generated header, BUILT_SOURCE).
- The copy does not have implications for linking (like mgcp_common.c did) nor
  is it a source for build confusion or fallout in other projects, because it
  does not reach across several git source trees (like gsm_data_shared.h did).

mgcp_connection_mode_strs are not actually used in libosmo-mgcp, so drop them.
(It would make semantic sense to have then in mgcp, but we can add it when it
is needed. A similar value string array remains in libosmo-mgcp-client.)

Change-Id: I7a5d3b9a2eb90be7e34b95efa529429f2e6c3ed8
diff --git a/include/Makefile.am b/include/Makefile.am
index 3cc4b1d..b52e5ea 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -9,6 +9,7 @@
 	osmocom/mgcp_client/mgcp_client.h \
 	osmocom/mgcp_client/mgcp_common.h \
 	osmocom/mgcp/mgcp.h \
+	osmocom/mgcp/mgcp_common.h \
 	osmocom/mgcp/mgcp_internal.h \
 	osmocom/mgcp/osmux.h \
 	$(NULL)