sockets: Rework unix sockets onto fdinfo scheme

This is a big change, yes. Dump unix sockets in the same manner
as all the other files are done now. A few notes however.

1. We explicitly drop names for connected stream sockets. This is
   done to avoid conflicts with names -- accepted sockets share their
   names with the listening parent. This can be done later by binding
   a socket to a name, them renaming it to some temporary uniq one
   and at the very very end renaming some back to original.

2. Interconnected sockets are restored via socketpair() call. This is
   correct, but names are dropped. Need to bind() sockets after this
   (yes, this can be done), but for this we need to implement the trick
   with renames described before.

3. FD for socket queues is constantly re-opened not to resolve fd
   conflicts. Need to use service fds engine for this later.

4. Some code cleanup is still required, yes (will follow shortly).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>