Move the Unix_configuration implementation to lwt/

Signed-off-by: David Scott <dave.scott@citrix.com>
Create 2 CLIs: "unixcat" and "xencat" which use different backends

unixcat uses mmap(2) for memory, Unix domain sockets for signalling
  and Unix environment variables for configuration

xencat uses gnttab/gntshr for memory, evtchn for signalling and
  Xenstore keys for configuration

We also add Vchan_lwt_io which creates Lwt_io channels from an
endpoint implementation. Vchan_lwt_unix is left for backwards
compatibility.

Signed-off-by: David Scott <dave.scott@citrix.com>