Make mount paths relative to rootfs

Make mount paths relative to rootfs

Why not chdir into the root of container right when
the root filesystem is (bind-)mounted, and let all
mount entries to be relative to the container root?

Even more, to warn if lxc.mount[.entry] contains
absolute path for the destination directory (or a
variation of this, absolute and does not start with
container root mount point)?

This way, all mounts will look much more sane, and
it will be much easier to move/clone containers -
by changing only lxc.rootfs.

I do it this way locally since the beginning, by
chdir'ing to the proper directory (rootfs) before
running lxc-start (in a startup script), but this
is now broken in 0.7.3 which bind-mounts rootfs
somewhere in /usr/lib/lxc.

Signed-off-by: Michael Tokarev<mjt@tls.msk.ru>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>