unbound user and group fix-hint at end of installation ./build.sh

[bch <brad.harder%gmail.com@localhost>]

At the end of ./build.sh, I got a note about needing to include
_unbound to user and group. Isn't that something that should just be
handled by etcupdate (in fact, it looks like etcupdate does know about
this)?  Why would we suggest an error-prone manual update when the
next step in the update (etcupdate) will do this for you in an
automated way ?

-bch