*: console rewrite

This implements {createTTY, detach} and all of the combinations and
negations of the two that were previously implemented. There are some
valid questions about out-of-OCI-scope topics like !createTTY and how
things should be handled (why do we dup the current stdio to the
process, and how is that not a security issue). However, these will be
dealt with in a separate patchset.

In order to allow for late console setup, split setupRootfs into the
"preparation" section where all of the mounts are created and the
"finalize" section where we pivot_root and set things as ro. In between
the two we can set up all of the console mountpoints and symlinks we
need.

We use two-stage synchronisation to ensures that when the syscalls are
reordered in a suboptimal way, an out-of-place read() on the parentPipe
will not gobble the ancilliary information.

This patch is part of the console rewrite patchset.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

create.go

@@ -29,11 +29,6 @@ command(s) that get executed on start, edit the args parameter of the spec. See
 			Value: "",
 			Usage: `path to the root of the bundle directory, defaults to the current directory`,
 		},
-		cli.StringFlag{
-			Name:  "console",
-			Value: "",
-			Usage: "specify the pty slave path for use with the container",
-		},
 		cli.StringFlag{
 			Name:  "pid-file",
 			Value: "",