MEDIUM: sockpair: implement sockpair_bind_receiver()

MEDIUM: sockpair: implement sockpair_bind_receiver()

Note that for now we don't have a sockpair.c file to host that unusual
family, so the new function was placed directly into proto_sockpair.c.
It's no big deal given that this family is currently not shared with
multiple protocols.

The function does almost nothing but setting up the receiver. This is
normal as the socket the FDs are passed onto are supposed to have been
already created somewhere else, and the only usable identifier for such
a socket pair is the receiving FD itself.

The function was assigned to sockpair's ->bind() and is not used yet.