Introduce VOP_UNP_BIND(), VOP_UNP_CONNECT(), and VOP_UNP_DETACH()
operations for setting and accessing vnode's v_socket field.

The operations are necessary to implement proper unix socket handling
on layered file systems like nullfs(5).

This change fixes the long standing issue with nullfs(5) being in that
unix sockets did not work between lower and upper layers: if we bound
to a socket on the lower layer we could connect only to the lower
path; if we bound to the upper layer we could connect only to the
upper path. The new behavior is one can connect to both the lower and
the upper paths regardless what layer path one binds to.

PR: kern/51583, kern/159663
Suggested by: kib
Reviewed by: arch
MFC after: 2 weeks