C: revert *-,*+,*! fetch magic, assign all core GVs to their global symbols

*!+- gv_fetch loading needs *INC, but this is not setup yet. so preload it again
at compile-time. Fixes 901, 902, 903 (#199) but not 904 (#220) yet

alias the proper global GVs: envgv, argvgv, incgv, stdingv, stderrgv, hintgv,
defgv, errgv, replgv.
strip defstash "main::" from gv_fetchpv calls

fix sv_dump unthreaded
more #201 testcases