check whether rootfs is shared before running pre-mount hooks

this expands c597baa8f9 and 2c6f3fc932.

Also move the block using detect_ramfs_rootfs() from setup_rootfs() to
lxc_setup()

Signed-off-by: Florian Klink <flokli@flokli.de>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
move lxc-init to /sbin/init.lxc

Using the multiarch dir causes problems when running lxc-execute
on amd64 with an i386 container.  /sbin/lxc-init is a more confusing
name and will show up in 'lxc<tab>'.  /sbin/init.lxc should be quite
obvious as an init for lxc.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>