C: fix #289, add GV->CV backref magic

simplify CvFLAGS handling to filter out CVf_DYNFILE and CVf_CVGV_RC
fix too high REFCNT for CVs (cross-checked with pure-perl)
run-time CvCVGV_RC_off for backref magic CVs

now we have new cv_undef failures to investigate (perlio, attributes, ...)