Bluetooth: userchan: Support other libCs

Refactor the userchan driver into a top and a bottom part.
The bottom is the one which interacts with the host and is built
with the host libC, while the top is built with the embedded code
and whatever libC that is built with.

Errors (errno) is converted between the top and bottom to ensure they
are coherent with the local libC.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>