Set default paths for unprivileged use

When running unprivileged (euid != 0), LXC will now use the following paths:
 - Default lxc path: ~/.local/share/lxc/
 - Default config path: ~/.config/lxc/lxc.conf

Those two paths are based on standard XDG paths (though ignoring all the
possible override paths for now at least) and so probably don't need to
be configurable at build time.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>