revert respecting userpriv during pkg_setup

FEATURES=userpriv is not honored for pkg_setup.
use correct function name
drop redundant arch use flag addition to the enabled use flag set

The arch related use flag is already added to the forced use flags via
use.force in the profile and via code directly below.

This also fixes an issue where use expand settings in make.conf were
being overridden by profile settings, specifically an amd64 profile
level python_targets_python3_2 disable is overriding a local make.conf
setting.
fix support for profile-based stable use masking and forcing

Fixes issues causing the related profile files not to be parsed at all
and adds support for the use.stable.force and use.stable.mask files in
addition to the package specific versions.