Fixes#477
Because we start piping stdin before process launch we can fill up the
guest pipe buffer before the process even starts. We'd need some
backpressure mechanism to handle this and slow consumers (register the
other end with epoll and buffer some data etc.), but we should probably
just start piping after the process is up and running. This change does
exactly that, as well as stops holding the process mutex while draining
stdin in the guest for `CloseStdin()`.
This fixes issues where we try and write > pipe_buf bytes through stdin.
Today this hangs.
Danny Canter
26f3dcc796