[BREAK] merge MultiArch-ABI. We don't support MultiArch,

[BREAK] merge MultiArch-ABI. We don't support MultiArch,
but we support the usage of the new ABI so libapt users
can start to prepare for MultiArch (Closes: #536029)

MultiArch isn't ready for Primetime usage for now, but the branch has
managed to be a NOP if used in SingleArch-mode so we can start to
promote the use of the new MultiArchable API-extensions.