close: set our 'live' flag to 'Exiting' and signal the event channel
close: set our 'live' flag to 'Exiting' and signal the event channel

This is the needed for the other end of the communication to detect that we
have shutdown, and it should return `Eof.

This is part of #24

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

djs55/ocaml-vchan,mirage/ocaml-vchan,samoht/ocaml-vchan,mirage/ocaml-vchan,djs55/ocaml-vchan,djs55/ocaml-vchan,samoht/ocaml-vchan,mirage/ocaml-vchan,samoht/ocaml-vchan