Handle userns isolation and dropping root at the same time

passt/pasta can interact with user namespaces in a number of ways:
   1) With --netns-only we'll remain in our original user namespace
   2) With --userns or a PID option to pasta we'll join either the given
      user namespace or that of the PID
   3) When pasta spawns a shell or command we'll start a new user namespace
      for the command and then join it
   4) With passt we'll create a new user namespace when we sandbox()
      ourself

However (3) and (4) turn out to have essentially the same effect.  In both
cases we create one new user namespace.  The spawned command starts there,
and passt/pasta itself will live there from sandbox() onwards.

Because of this, we can simplify user namespace handling by moving the
userns handling earlier, to the same point we drop root in the original
namespace.  Extend the drop_user() function to isolate_user() which does
both.

After switching UID and GID in the original userns, isolate_user() will
either join or create the userns we require.  When we spawn a command with
pasta_start_ns()/pasta_setup_ns() we no longer need to create a userns,
because we're already made one.  sandbox() likewise no longer needs to
create (or join) an userns because we're already in the one we need.

We no longer need c->pasta_userns_fd, since the fd is only used locally
in isolate_user().  Likewise we can replace c->netns_only with a local
in conf(), since it's not used outside there.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

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