Fix handling of empty config keys

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
lxc-clone: support unprivileged use

This also fixes unprivileged use of lxc-snapshot and lxc-rename.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
doc: Try to clear some confusion about lxc.conf

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