sockstat(1): print out full connection graph for unix(4) sockets

Kernel provides us with enough information to display all possible
connections between UNIX sockets.

o Store unp_conn, xu_firstref and xu_nextref in the faddr of a UNIX sock.
o Build tree of file descriptors, indexed by the socket pointer.
o In displaysock() print out all possible information:

1. if socket is bound, print name of this socket
2. if socket has connected to a peer with a name, print peers name
3. if socket has connected to a peer without a name, print [pid fd]
4. if a bound socket has received connections, print list of them as [pid fd] Previously, only 1) either 2) were printed.

Reviewed by: tuexen
Differential revision: https://reviews.freebsd.org/D35726