Let listen() return EDESTADDRREQ when not bound.

We currently return EINVAL when calling listen() on a UNIX socket that
has not been bound to a pathname. If my interpretation of POSIX is
correct, we should return EDESTADDRREQ: "The socket is not bound to a
local address, and the protocol does not support listening on an unbound
socket."

Return EDESTADDRREQ instead when not bound and not connected.

Differential Revision: https://reviews.freebsd.org/D3038
Reviewed by: gnn, network