container creation: support unpriv container creation in user namespaces

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>