C: use GV_NOTQUAL and GV_ADD|GV_ADDMULTI
C: skip newGP, fixup GV_ADDMULTI

newGP is already done via gv_fetchpv GV_ADD (at least >5.10)
and is problematic with already existing GPs.

Need to check earlier perls
Need to check existing GvFLAGS and translate it to gv_fetchpv flags
(ADDMULTI and what else?)
C: do not skip bareword GVs only, bareword IO handles

add -fstash pod