Introduce a first set of container hooks

Introduce a first set of container hooks

This patch introduces support for 4 hooks.  We'd like to have 6 in
all to mirror the openvz ones (thanks to Stéphane for this info):

pre-start: in the host namespace before container mounting happens
mount: after container mounting (as per config and /var/lib/lxc/container/fstab)
       but before pivot_root
start: immediately before exec'ing init
stop: in container namespace and in chroot before shutdown
umount: after other unmounting has happened
post-stop: outside of the container

stop and umount are not implemented here because when the kernel kills
the container init, it kills the namespace.  We can probably work around
this, i.e. by keeping the /proc/pid/ns/mnt open, and using that, though
all container tasks including init would still be dead.  Is that worth
pursuing?

start also presents a bit of an issue.  openvz allows a script on the
host to be specified, apparently.  My patch requires the script or
program to exist in the container.  I'm fine with trying to do it the
openvz way, but I wasn't sure what the best way to do that was.  Openvz
(I'm told) opens the script and passes its contents to a bash in the
container.  But that limits the hooks to being only scripts.  By
requiring the hook to be in the container, we can allow any sort of
hook, and assume that any required libraries/dependencies exist
there.

Other than that with this patchset I can add

lxc.hook.pre-start = /var/lib/lxc/p1/pre-start
lxc.hook.mount = /var/lib/lxc/p1/mount
lxc.hook.start = /start
lxc.hook.post-stop = /var/lib/lxc/p1/post-stop

to my /var/lib/lxc/p1/config, and the hooks get executed as expected.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>