add lxc_clone.c

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
only replace lxcname/lxcpath as wholeword

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
support overlayfs snapshto of dir in lxc_clone

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>