spin: Update to most recent version. (thanks Ori_B)

from Ori_B:

There were a small number of changes needed from the tarball
on spinroot.org:

  - The mkfile needed to be updated
  - Memory.h needed to not be included
  - It needed to invoke /bin/cpp instead of gcc -E
  - It depended on `yychar`, which our yacc doesn't
    provide.

I'm still figuring out how to use spin, but it seems to do
the right thing when testing a few of the examples:

	% cd $home/src/Spin/Examples/
	% spin -a peterson.pml
	% pcc pan.c -D_POSIX_SOURCE
	% ./6.out

	(Spin Version 6.4.7 -- 19 August 2017)
		+ Partial Order Reduction

	Full statespace search for:
		never claim         	- (none specified)
		assertion violations	+
		acceptance   cycles 	- (not selected)
		invalid end states	+

	State-vector 32 byte, depth reached 24, errors: 0
	40 states, stored
	27 states, matched
	67 transitions (= stored+matched)
		0 atomic steps
	hash conflicts:         0 (resolved)

	Stats on memory usage (in Megabytes):
	0.002	equivalent memory usage for states (stored*(State-vector + overhead))
	0.292	actual memory usage for states
	128.000	memory used for hash table (-w24)
	0.534	memory used for DFS stack (-m10000)
	128.730	total actual memory usage


	unreached in proctype user
		/tmp/Spin/Examples/peterson.pml:20, state 10, "-end-"
		(1 of 10 states)

	pan: elapsed time 1.25 seconds
	pan: rate        32 states/second

1 files changed, 21 insertions, 12 deletions

diff --git a/sys/src/cmd/spin/tl_rewrt.c b/sys/src/cmd/spin/tl_rewrt.c
index 6e00d9ac0..ddc437260 100644
--- a/sys/src/cmd/spin/tl_rewrt.c
+++ b/sys/src/cmd/spin/tl_rewrt.c
@@ -1,16 +1,13 @@
 /***** tl_spin: tl_rewrt.c *****/
 
-/* Copyright (c) 1995-2003 by Lucent Technologies, Bell Laboratories.     */
-/* All Rights Reserved.  This software is for educational purposes only.  */
-/* No guarantee whatsoever is expressed or implied by the distribution of */
-/* this code.  Permission is given to distribute this code provided that  */
-/* this introductory message is not removed and no monies are exchanged.  */
-/* Software written by Gerard J. Holzmann.  For tool documentation see:   */
-/*             http://spinroot.com/                                       */
-/* Send all bug-reports and/or questions to: bugs@spinroot.com            */
-
-/* Based on the translation algorithm by Gerth, Peled, Vardi, and Wolper, */
-/* presented at the PSTV Conference, held in 1995, Warsaw, Poland 1995.   */
+/*
+ * This file is part of the public release of Spin. It is subject to the
+ * terms in the LICENSE file that is included in this source directory.
+ * Tool documentation is available at http://spinroot.com
+ *
+ * Based on the translation algorithm by Gerth, Peled, Vardi, and Wolper,
+ * presented at the PSTV Conference, held in 1995, Warsaw, Poland 1995.
+ */
 
 #include "tl.h"
 
@@ -18,6 +15,12 @@ extern int	tl_verbose;
 
 static Node	*can = ZN;
 
+void
+ini_rewrt(void)
+{
+	can = ZN;
+}
+
 Node *
 right_linked(Node *n)
 {
@@ -137,6 +140,9 @@ addcan(int tok, Node *n)
 	}
 
 	s = DoDump(N);
+	if (!s)
+	{	fatal("unexpected error 6", (char *) 0);
+	}
 	if (can->ntyp != tok)	/* only one element in list so far */
 	{	ptr = &can;
 		goto insert;
@@ -146,7 +152,10 @@ addcan(int tok, Node *n)
 	prev = ZN;
 	for (m = can; m->ntyp == tok && m->rgt; prev = m, m = m->rgt)
 	{	t = DoDump(m->lft);
-		cmp = strcmp(s->name, t->name);
+		if (t != ZS)
+			cmp = strcmp(s->name, t->name);
+		else
+			cmp = 0;
 		if (cmp == 0)	/* duplicate */
 			return;
 		if (cmp < 0)