container creation: support unpriv container creation in user namespaces

1. lxcapi_create: don't try to unshare and mount for dir backed containers

It's unnecessary, and breaks unprivileged lxc-create (since unpriv users
cannot yet unshare(CLONE_NEWNS)).

2. api_create: chown rootfs

chown rootfs to the host uid to which container root will be mapped

3. create: run template in a mapped user ns

4. use (setuid-root) newxidmap to set id_map if we are not root

This is needed to be able to set userns mappings as an unprivileged
user, for unprivileged lxc-start.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
fix chowning of tty and console uids

It needs to be done from the handler, not the container, since
the container may not have the rights.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>

Changelog:
    Jul 22: remove hardcoded path for /bin/chown
    Jul 22: use new lxc-usernsexec

Conflicts:
	src/lxc/lxccontainer.c
lxc-busybox: if in userns, don't try to mknod

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
start: use lxc-user-nic if we are not root

Note this results in nics named things like 'lxcuser-0p'.  We'll
likely want to pass the requested name to lxc-user-nic, but let's
do that in a separate patch.

If we're not root, we can't create new network itnerfaces to pass
into the container.  Instead wait until the container is started,
and call lxc-user-nic to create and assign the nics.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>