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Hex-encode non-ASCII random seeds in save files

The developer documentation claims that save files are long ASCII
strings.  This is mostly true, but there's nothing stopping a user
from entering non-ASCII characters as random seeds.  The ASCII
property of save files is useful, so encode seeds in hex before
writing them unless they consist only of printable ASCII characters.

Hex-encoded seeds are written under a new key, HEXSEED, to distinguish
them from unencoded seeds.  This means that old versions of the code
won't be able to load encoded seeds, but that's not a great loss:
seeds aren't generally portable between versions anyway.

--- a/midend.c

+++ b/midend.c

@@ -2061,8 +2061,27 @@

     /*

      * The current game description, the privdesc, and the random seed.

      */

-    if (me->seedstr)

-        wr("SEED", me->seedstr);

+    if (me->seedstr) {

+        /*

+         * Random seeds are not necessarily printable ASCII.

+         * Hex-encode the seed if necessary.  Printable ASCII seeds

+         * are emitted unencoded for compatibility with older

+         * versions.

+         */

+        int i;

+

+        for (i = 0; me->seedstr[i]; i++)

+            if (me->seedstr[i] < 32 || me->seedstr[i] >= 127)

+                break;

+        if (me->seedstr[i]) {

+            char *hexseed = bin2hex((unsigned char *)me->seedstr,

+                                    strlen(me->seedstr));

+

+            wr("HEXSEED", hexseed);

+            sfree(hexseed);

+        } else

+            wr("SEED", me->seedstr);

+    }

     if (me->desc)

         wr("DESC", me->desc);

     if (me->privdesc)

@@ -2264,6 +2283,15 @@

                 sfree(data.cparstr);

                 data.cparstr = val;

                 val = NULL;

+            } else if (!strcmp(key, "HEXSEED")) {

+                unsigned char *tmp;

+                int len = strlen(val) / 2;   /* length in bytes */

+                tmp = hex2bin(val, len);

+                sfree(data.seed);

+                data.seed = snewn(len + 1, char);

+                memcpy(data.seed, tmp, len);

+                data.seed[len] = '\0';

+                sfree(tmp);

             } else if (!strcmp(key, "SEED")) {

                 sfree(data.seed);

                 data.seed = val;