qa/common.check: rework _change_config() for Gentoo
qa/common.check: rework _change_config() for Gentoo

on/off ... => unconditional
    # rc-update add/delete ... default

This helps qa/964 pass on vm05 and makes no difference to
any non-Gentoo platform.

wuliming/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,aeg-aeg/pcpfans,wuliming/pcp,wuliming/pcp,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,aeg-aeg/pcpfans