kernel-install: avoid using 'cp --preserve'

Force 0644 and root:root instead, to avoid problems with fat filesystems.
Add a bit more explicit message, to help confused users

Seeing http://www.happyassassin.net/2013/09/27/further-sysadmin-adventures-wheres-my-freeipa-badge/
it seems that the default message is a bit confusing for people
who never encountered it before, so adding a link to the manpage could
help them.