confile.c: clear entries if no value

For list configuration entries like capabilities and cgroups
entries, if there is a 'key =' value (i.e. "lxc.cap.drop =")
then clear any loaded entries.

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