add note about bash join_by function
add example upgrading script
fix duplicate package upgrade lines

it is common practice to have multiple kernel-* packages installed to
have a fallback version. while upgrading, we only need to 'upgrade'
_one_ of the installed packages of the same name.

nb: it may be worth a thought to rewrite the whole package upgrading, but
it is fast enough for now, so that the double loop isn't too painful for
remove check for uid/gid in FILELIST.TXT