MINOR: sockpair: implement sockpair_accept_conn() to accept a connection

MINOR: sockpair: implement sockpair_accept_conn() to accept a connection

This is the same as previous commit, but this time for the sockpair-
specific stuff, relying on recv_fd_uxst() instead of accept(), so the
code is simpler. The various errno cases are handled like for regular
sockets, though some of them will probably never happen, but this does
not hurt.