Problem with apply-frame trap in 1.6 and 1.7

[Neil Jerram]

Working on breakpoints for 1.6.x, I just discovered that the  following 
ENTER_APPLY trap code in eval.c goes into a tight busy loop if 
(debug-enable 'trace) and (trap-set! apply-frame-handler non-#f).

 if (CHECK_APPLY && SCM_TRAPS_P)\
   if (SCM_APPLY_FRAME_P || (SCM_TRACE_P && PROCTRACEP (proc)))\

This is because PROCTRACEP uses scm_procedure_property, which itself 
uses apply if proc is not a closure.

I think the fix (which works for me in 1.6.5) is to redefine PROCTRACEP 
to use a more targeted function, like this:

#define PROCTRACEP(x) closure_traced_p (x)

SCM closure_traced_p (SCM p)
{
 if (!SCM_CLOSUREP (p))
   return 0;
 return SCM_NIMP (scm_sloppy_assq (scm_sym_trace, SCM_PROCPROPS (p)));
}

Thoughts?  Once the fix is agreed, I suggest that this should go into 
1.6.x as well as HEAD.

Regards,
          Neil