This is a proof-of-concept for the --console-socket API. It just acts as
a dumb input-output copy process (nowhere near as good as the internal
runC one since it doesn't handle console resizes or signals). It also
provides a test-friendly mode that will be used in the bats integration
tests.
This patch is part of the console rewrite patchset.
Signed-off-by: Aleksa Sarai <asarai@suse.de>