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.