mgcp: Introduce a mgcp_trunk_config enum for endpoint configs

We want to support real trunks in the MGCP code and we need to
have some better book keeping for those. Move the code around.
diff --git a/openbsc/tests/bsc-nat/bsc_nat_test.c b/openbsc/tests/bsc-nat/bsc_nat_test.c
index 3e943cf..504b691 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.c
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.c
@@ -446,7 +446,7 @@
 					       struct bsc_endpoint,
 					       65);
 	nat->mgcp_cfg = mgcp_config_alloc();
-	nat->mgcp_cfg->number_endpoints = 64;
+	nat->mgcp_cfg->trunk.number_endpoints = 64;
 
 	bsc = bsc_connection_alloc(nat);
 	bsc->cfg = bsc_config_alloc(nat, "foo");
@@ -491,7 +491,7 @@
 					       struct bsc_endpoint,
 					       33);
 	nat->mgcp_cfg = mgcp_config_alloc();
-	nat->mgcp_cfg->number_endpoints = 64;
+	nat->mgcp_cfg->trunk.number_endpoints = 64;
 
 	bsc = bsc_connection_alloc(nat);
 	bsc->cfg = bsc_config_alloc(nat, "foo");