Per-thread magicals now stored in their own thr->magicals and keyed

Per-thread magicals now stored in their own thr->magicals and keyed
more directly. cvcache and oursv become ordinary struct thread
fields instead of #defined thr->Tfoo ones. SvREFCNT_inc now checks
for 0 again. Main thread initialisation done by new function
init_main_thread instead of (now fixed) new_struct_thread.

p4raw-id: //depot/perl@205