t/c_allow_opts.t: a => allow for parallel testing
C shared-gp: need to share GPs for stashes

honor shared GPs, no recreation (%gptable).
do not create GPs for stashes at all.

fixes issues #212, #209, #210.
breaks C -O0: 29,41,45,46,68,91,95,125
C shared-gp: fix $gpp, use numeric context 0+$gp instead

fixes the failing tests
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?)