C: fix #126 Do not reset existing CVs

Do not reset the GvCV of already booted core XS functions.
Backpatch the init section for those few functions (re, mro, UNIVERSAL, ...)

Fixed Conflicts:
	lib/B/C.pm
BC: fix tests 42,43 Attempt to free unreferenced scalar

push_begin resetting xcv_gv to NULL is invalid with CvGV_set