Changes

MGW control: migrate to new API for codecs and payload types

struct mgcp_conn_peer in libosmo-mgcp-client API used to have a
mandatory codecs[] array for codec specification and an optional
ptmap[] array to be used only when the default payload type needs
to be overridden.  But now the codecs[] array is deprecated,
and new API features (see following patches) require the use of
ptmap[] array, with mandatory specification of payload type by
the MGCP client.  Convert osmo-bsc to this new API.

Change-Id: Iafc38a3da64ce7c2f060a32864174dcde9f57b56