Make pipes use only one channel of each type

[lw]

Currently the two endpoints of a pipe open one instance of all the channel types they have in common. However, due to a strict priority order between them, they will inevitably end up using only one of them (for each type: CPU, CUDA, ...) leaving the other ones unused. We should avoid opening them in the first place. This may also simplify a bit the content of the libnop messages.