* Implement SOCK_SEQPACKET sockets for local communications. These sockets

* Implement SOCK_SEQPACKET sockets for local communications.  These sockets
  operate like SOCK_STREAM but each write() builds a record and each read()
  reads a record.  That is, the data is not aggregated together or allowed
  to be partially read.

  This allows local sockets to have the same packetization characteristics
  as if_tap when desired.

* Add a feature to the vkernel which allows a unix domain socket to be
  specified for the network interface rather then a TAP interface.  The
  vkernel will connect to the socket using SOCK_SEQPACKET and read and
  write packets to it.

* Clean up some libc/kernel namespace collisions related to including
  sys/socket.h.