mount /proc/xen in init.d/xen

mount /proc/xen in init.d/xen

pvops dom0 kernels have a separate xenfs which has to be mounted on
/proc/xen.  Systems with older configurations don't have xenfs listed
in fstab, and it can sometimes make sense to keep it that way (for
example, if the dom0 wants to boot a native-only kernel too).

The attached patch to the script which ends up in /etc/init.t/xend
mounts /proc/xen if it appears to be necessary.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>