pp.c & sv.c: pp_ref UTF8 and null cleanup.

pp.c & sv.c: pp_ref UTF8 and null cleanup.

This adds a new function to sv.c, sv_ref, which is a nul-and-UTF8
clean version of sv_reftype. pp_ref now uses that.

sv_ref() not only returns the SV, but also takes in an SV
to modify, so we can say both sv_ref(TARG, obj, TRUE); and
sv = sv_ref(NULL, obj, TRUE);