Changes

mgw: Introduce struct mgcp_codecset struct

We do tons of operations on 3 fields (array of codecs, len of array,
selected code) which can be isolated.
Right now, we are using APIs which somehow require structs 2-3 levels of
encapsulation above the ones really required, which makes all code
totally entangled, difficult to understand and prone to errors when
changing stuff in deeply nested structs.
A prove of this is how this patch affects a lot of lines in lots of
places.

Change-Id: Id7db7ab01d56b7fa2415123b604375e48c82ab25