Rename 'Connection' to 'Endpoint'

So now we have

  module V = Vchan.Endpoint.Make(...) : Vchan.S.ENDPOINT
  let client = V.client ...
  let server = V.server ...

Signed-off-by: David Scott <dave.scott@citrix.com>
Replace another Vchan.S.S with Vchan.S.ENDPOINT

Signed-off-by: David Scott <dave.scott@citrix.com>
Make {read,write}_size optional; and adopt a safe default of 1024

Ideally we would use a larger buffer size by default but this would
require multiple grants, and some contemporary linux kernel
xen-gnt{dev,alloc} devices can't handle multiple page grants/maps.

Signed-off-by: David Scott <dave.scott@citrix.com>
unix: rename Connection to Endpoint

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #42 from djs55/interface-tweak

Rename 'Connection' to 'Endpoint'