microcode/unix: fix continue_from_trap to freak out (force GC) less

microcode/unix: fix continue_from_trap to freak out (force GC) less

The classify_pc function returns pcl_unknown for traps in shared
libraries, e.g. in feraiseexcept (in the raise-float-exceptions
primitive) or any callout (in the c-call primitive).  This new
continue_from_trap function is less conservative, ignoring the
pcl_unknown result from classify_pc when the stack_pointer, Free and
PRIMITIVE registers all look good.  This eliminated all complaints
during `make check' about "resetting bogus Free" (not the actual
problem), representing 14 unnecessarily forced GCs.

Also, fix the murmur about "resetting bogus Free".  Correct it for
accuracy and so that it must be turned on with the
((make-primitive-procedure 'SET-DEBUG-FLAGS! 2) 9 #t)