libbrillo: allow redirecting process input

Add a RedirectInput method to Process that allows redirecting
stdin to read from a file.

BUG=chromium:767088
TEST=unittests

Change-Id: Id3d440048792a9c89e1ffb3b59ba26f888191678
Reviewed-on: https://chromium-review.googlesource.com/673355
Commit-Ready: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
4 files changed