Remove /etc/passwd and /etc/group parsing on runc run/exec

[kolyshkin]

1. libct: switch to numeric UID/GID/groups

   This addresses the following TODO in the code (added back in 2015
   by commit 845fc65):

   // 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.

2. libct: earlier Rootless vs AdditionalGroups check

   Since the UID/GID/AdditonalGroups fields are now numeric,
   we can address the following TODO item in the code (added
   by commit d2f4969 back in 2016):

   TODO: We currently can't do
   this check earlier, but if libcontainer.Process.User was typesafe
   this might work.

   Move the check to much earlier phase, when we're preparing
   to start a process in a container.

3. runc list: use standard os/user

   Switch from github.com/moby/sys/user to Go stdlib os/user
   (which has both libc-backed and pure Go implementations).

4. libct/user: remove

   This package is marked as deprecated in v1.2.0, and have no internal
   users now, so let's remove it (for v1.3.0).

Fixes: #3998

[thaJeztah]

• related: libcontainer/userns: simplify, and separate from "user" package. #2868

[kolyshkin]

This is still a draft pending #4325 merge -- once that one is merged, I will rework setUser here.

[kolyshkin]

This seems to be ready now. @AkihiroSuda @cyphar @lifubang @rata PTAL. Overall I think it's a big step forward:

• addressing a couple of (rather glaring) TODOs from 2015/2016;
• straightening libct API;
• removing code that shouldn't be there and does nothing;
• all without breaking runc's backward compatibility (hopefully).

Also, I know that the second commit is a bit too big for review. I tried to split it into two but that seems unfeasible, as the two features (changing user string to uid:gid and moving the setting of HOME to prepareEnv) are interdependent.