CLI: rename to 'xencat' and make the arguments similar to 'nc'

If you want to connect (as a client) to a remote domain use

  xencat <domid> [path]
or
  xencat <domid> /some/path/where/server/lives

If you want to listen to a remote domain (NB you have to specify one
in order to grant the necessary permissions) use

  xencat -l <domid> [path]
or
  xencat -l <domid> /some/path/where/server/lives

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