Lwt_io.open_file: warn about truncation by default
Lwt_io.open_file: warn about truncation by default

Resolves #570.

[skip ci]

rneswold/lwt,ocsigen/lwt,ocsigen/lwt,rneswold/lwt,c-cube/lwt,c-cube/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt