libct: switch to numeric UID/GID/groups

This addresses the following TODO in the code (added back in 2015
by commit 845fc65e5):

> // TODO: fix libcontainer's API to better support uid/gid in a typesafe way.

Historically, libcontainer internally uses strings for user, group, and
additional (aka supplementary) groups.

Yet, runc receives those credentials as part of runtime-spec's process,
which uses integers for all of them (see [1], [2]).

What happens next is:

1. runc start/run/exec converts those credentials to strings (a User
   string containing "UID:GID", and a []string for additional GIDs) and
   passes those onto runc init.
2. runc init converts them back to int, in the most complicated way
   possible (parsing container's /etc/passwd and /etc/group).

All this conversion and, especially, parsing is totally unnecessary,
but is performed on every container exec (and start).

The only benefit of all this is, a libcontainer user could use user and
group names instead of numeric IDs (but runc itself is not using this
feature, and we don't know if there are any other users of this).

Let's remove this back and forth translation, hopefully increasing
runc exec performance.

The only remaining need to parse /etc/passwd is to set HOME environment
variable for a specified UID, in case $HOME is not explicitly set in
process.Env. This can now be done right in prepareEnv, which simplifies
the code flow a lot. Alas, we can not use standard os/user.LookupId, as
it could cache host's /etc/passwd or the current user (even with the
osusergo tag).

PS Note that the structures being changed (initConfig and Process) are
never saved to disk as JSON by runc, so there is no compatibility issue
for runc users.

Still, this is a breaking change in libcontainer, but we never promised
that libcontainer API will be stable (and there's a special package
that can handle it -- github.com/moby/sys/user). Reflect this in
CHANGELOG.

For 3998.

[1]: https://github.com/opencontainers/runtime-spec/blob/v1.0.2/config.md#posix-platform-user
[2]: https://github.com/opencontainers/runtime-spec/blob/v1.0.2/specs-go/config.go#L86

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/process.go

@@ -26,13 +26,13 @@ type Process struct {
 	// Env specifies the environment variables for the process.
 	Env []string
 
-	// User will set the uid and gid of the executing process running inside the container
+	// UID and GID of the executing process running inside the container
 	// local to the container's user and group configuration.
-	User string
+	UID, GID int
 
 	// AdditionalGroups specifies the gids that should be added to supplementary groups
 	// in addition to those that the user belongs to.
-	AdditionalGroups []string
+	AdditionalGroups []int
 
 	// Cwd will change the processes current working directory inside the container's rootfs.
 	Cwd string