Log Message:
Make it easier to build and install packages "unprivileged", where
the owner of all installed files is a non-root user.  This change
affects most packages that require special users or groups by making
them use the specified unprivileged user and group instead.

(1) Add two new variables PKG_GROUPS_VARS and PKG_USERS_VARS to
    unprivileged.mk.  These two variables are lists of other bmake
    variables that define package-specific users and groups.  Packages
    that have user-settable variables for users and groups, e.g. apache
    and APACHE_{USER,GROUP}, courier-mta and COURIER_{USER,GROUP},
    etc., should list these variables in PKG_USERS_VARS and PKG_GROUPS_VARS
    so that unprivileged.mk can know to set them to ${UNPRIVILEGED_USER}
    and ${UNPRIVILEGED_GROUP}.

(2) Modify packages to use PKG_GROUPS_VARS and PKG_USERS_VARS.