[CM] C Stack Unwinding

[bil at ccrma.Stanford.EDU]

There is a bug in s7 -- it did not protect against a stack underflow
because it incorrectly assumed the s7_call_with_catch was in a
context where some scheme code was awaiting a result, not all by itself
(hence the "unknown operator" or "stack underflow"). I need to test my
bugfix -- I will try to make a new version of s7 later today.

With that fixed, s7_call_with_catch returns #t because the error was
caught -- "ret" in bad_func is #f, bad_func displays "#f\n", then
returns #t.  So "result" is #t.

Thanks very much for the bug report!