- the socket stat counters have been moved from socket.h to stats.h.
- isc_nm_t now attaches to the same stats counter group as
  isc_socketmgr_t, so that both managers can increment the same
  set of statistics
- isc__nmsocket_init() now takes an interface as a paramter so that
  the address family can be determined when initializing the socket.
- based on the address family and socket type, a group of statistics
  counters will be associated with the socket - for example, UDP4Active
  with IPv4 UDP sockets and TCP6Active with IPv6 TCP sockets.  note
  that no counters are currently associated with TCPDNS sockets; those
  stats will be handled by the underlying TCP socket.
- the counters are not actually used by netmgr sockets yet; counter
  increment and decrement calls will be added in a later commit.