Modifies LISPFINISH subr to use 2nd argument, if present, for exit status (#525)

Redesign of exit status handling, instead of overloading the FAST flag
of (LOGOUT fast), add an optional 2nd argument, EXITSTATUS.

Retains compatability, with behavior unchanged, between new emulator
and older sysouts which do not pass the second argument to LISPFINISH.

Newer sysouts, which pass the 2nd argument, will continue to work with
older emulators though will not get the exit status set.

Author: nbriggs

src/subr.c

@@ -416,11 +416,11 @@ void OP_subrcall(int subr_no, int argnum) {
     case sb_LISP_FINISH: {
       int status;
       POP_SUBR_ARGS;
-      if (argnum == 0 || args[0] == NIL || args[0] == ATOM_T)
-        lisp_finish(EXIT_SUCCESS);
-      N_GETNUMBER(args[0], status, exit_fail);
+      if (argnum == 0 || argnum == 1 || (argnum == 2 && args[1] == NIL)) lisp_finish(EXIT_SUCCESS);
+      if (argnum > 2) lisp_finish(EXIT_FAILURE);
+      N_GETNUMBER(args[1], status, exit_fail);
       lisp_finish(status);
-      exit_fail:
+    exit_fail:
       lisp_finish(EXIT_FAILURE);
       break;
     }