Change pp_tie and pp_dbmopen to use perl_call_sv instead of a

Change pp_tie and pp_dbmopen to use perl_call_sv instead of a
DIY pp_entersub (in preparation for AUTOLOAD change). dbmopen
not tested. ofslen now maps to thr->Tofslen in thread.h. Added
missing #ifdef USE_THREADS around some DEBU_L statements in die().
Building without USE_THREADS fails quite a lot of tests. It looks
as though the move to per-thread magicals must be missing some
#ifdef USE_THREADS.

p4raw-id: //depot/perl@209