additional cbacktrace command (working patch)

define cbacktrace
  call cob_stack_trace(stderr)
end
document cbacktrace
  Print backtrace of all active COBOL runtime elements on stack.
  Usage: cbacktrace
end
alias cbt = cbacktrace
alias cwhere = cbacktrace

Note: a MI version should call this with GDB's stream "~" if possible to present the backtrace in the GDB output instead of on stderr.

Note2: GnuCOBOL 3.2 is intended to ship with a less verbose version, executable by:

set $cbacktrace_limit = 0
define cbacktrace
  set $count = $cbacktrace_limit
  if $argc == 1
    set $count = $arg0
  end
  call cob_backtrace(stderr, $count)
end
document cbacktrace
  Print backtrace of all active COBOL runtime elements on stack, or innermost COUNT elements.
  Usage: cbacktrace [COUNT]
  With a negative argument, print outermost -COUNT elements.
  Uses $cbacktrace_limit if no argument is given. A value of zero means unlimited levels. 
end
alias cbt = cbacktrace
alias cwhere = cbacktrace