implement backend drivers and container clone API (v3)

1. commonize waitpid users to use a single helper.  We frequently want
to run something in a clean namespace, or fork off a script.  This
lets us keep the function doing fork:(1)exec(2)waitpid simpler.

2. start a blockdev backend implementation.  This will be used for
mounting, copying, and snapshotting container filesystems.

3. implement btrfs, lvm, directory, and overlayfs backends.

4. For overlayfs, support a new lxc.rootfs format of
'bdevtype:<extra>'.  This means you can now use overlayfs-based
containers without using lxc-start-ephemeral, by using
lxc.rootfs = overlayfs:/readonly-dir:writeable-dir

5. add a set of simple clone testcases

6. Write a new lxc_clone.c based on api clone.

Still to do (there's more, but off top of my head):

1. support zfs, aufs
2. have clone handle other mount entries (right now it only clones
the rootfs)
3. python, lua, and go bindings (not me :)
4. lxc-destroy: if lvm backing store, check for snapshots of it.
   (what about directories which have overlayfs clones?)

Changes since v2:
	Initialize random generator when picking new macaddr (reported
	  by caglar@10ur.org)
	Fix wrong use of bitmask flags
	On copy-clone of btrfs, create a subvolume
	lxc_clone.c: respect the command line usage of the old script
	lxc-clone(1): update documentation
	Refuse to try changing backing stores expect to overlayfs, as
	  it is not implemented (yet) anyway.

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