Revamp of the Windows command-line parsing and puzzle-loading code.

The Windows puzzles now accept similar command-line syntax to the GTK
ones, in that you can give them either a game ID (descriptive, random
or just plain params) or the name of a save file. Unlike the GTK ones,
however, the save file interpretation is tried first; this is because
some puzzles (e.g. Black Box) will interpret any old string as a valid
(if boring) game ID, and unlike the GTK puzzles it's not feasible to
require users to disambiguate via a command-line option, because on
Windows a thing that might easily happen is that a user passes a save
file to a puzzle binary via 'Open With' in the GUI shell, where they
don't get the chance to add extra options.

In order to make this work sensibly in the all-in-one Windows app, I
had to get round to another thing I've been planning to do for a
while, which is to write a function to examine a saved game file and
find out which puzzle it's for. So the combined Windows binary will
auto-switch to the right game if you pass a save file on its command
line, and also if you use Load while the program is running.

Another utility function I needed is one to split the WinMain single
command line string into argv. For this I've imported a copy of
split_into_argv() from Windows PuTTY (which doesn't affect this
package's list of copyright holders, since that function was all my
own code anyway).

[originally from svn r9749]

midend.c

@@ -172,6 +172,11 @@ midend *midend_new(frontend *fe, const game *ourgame,
     return me;
 }
 
+const game *midend_which_game(midend *me)
+{
+    return me->ourgame;
+}
+
 static void midend_purge_states(midend *me)
 {
     while (me->nstates > me->statepos) {
@@ -1949,6 +1954,111 @@ char *midend_deserialise(midend *me,
     return ret;
 }
 
+/*
+ * This function examines a saved game file just far enough to
+ * determine which game type it contains. It returns NULL on success
+ * and the game name string in 'name' (which will be dynamically
+ * allocated and should be caller-freed), or an error message on
+ * failure.
+ */
+char *identify_game(char **name, int (*read)(void *ctx, void *buf, int len),
+                    void *rctx)
+{
+    int nstates = 0, statepos = -1, gotstates = 0;
+    int started = FALSE;
+
+    char *val = NULL;
+    /* Initially all errors give the same report */
+    char *ret = "Data does not appear to be a saved game file";
+
+    *name = NULL;
+
+    /*
+     * Loop round and round reading one key/value pair at a time from
+     * the serialised stream, until we've found the game name.
+     */
+    while (nstates <= 0 || statepos < 0 || gotstates < nstates-1) {
+        char key[9], c;
+        int len;
+
+        do {
+            if (!read(rctx, key, 1)) {
+                /* unexpected EOF */
+                goto cleanup;
+            }
+        } while (key[0] == '\r' || key[0] == '\n');
+
+        if (!read(rctx, key+1, 8)) {
+            /* unexpected EOF */
+            goto cleanup;
+        }
+
+        if (key[8] != ':') {
+            if (started)
+                ret = "Data was incorrectly formatted for a saved game file";
+	    goto cleanup;
+        }
+        len = strcspn(key, ": ");
+        assert(len <= 8);
+        key[len] = '\0';
+
+        len = 0;
+        while (1) {
+            if (!read(rctx, &c, 1)) {
+                /* unexpected EOF */
+                goto cleanup;
+            }
+
+            if (c == ':') {
+                break;
+            } else if (c >= '0' && c <= '9') {
+                len = (len * 10) + (c - '0');
+            } else {
+                if (started)
+                    ret = "Data was incorrectly formatted for a"
+                    " saved game file";
+                goto cleanup;
+            }
+        }
+
+        val = snewn(len+1, char);
+        if (!read(rctx, val, len)) {
+            if (started)
+            goto cleanup;
+        }
+        val[len] = '\0';
+
+        if (!started) {
+            if (strcmp(key, "SAVEFILE") || strcmp(val, SERIALISE_MAGIC)) {
+                /* ret already has the right message in it */
+                goto cleanup;
+            }
+            /* Now most errors are this one, unless otherwise specified */
+            ret = "Saved data ended unexpectedly";
+            started = TRUE;
+        } else {
+            if (!strcmp(key, "VERSION")) {
+                if (strcmp(val, SERIALISE_VERSION)) {
+                    ret = "Cannot handle this version of the saved game"
+                        " file format";
+                    goto cleanup;
+                }
+            } else if (!strcmp(key, "GAME")) {
+                *name = dupstr(val);
+                ret = NULL;
+                goto cleanup;
+            }
+        }
+
+        sfree(val);
+        val = NULL;
+    }
+
+    cleanup:
+    sfree(val);
+    return ret;
+}
+
 char *midend_print_puzzle(midend *me, document *doc, int with_soln)
 {
     game_state *soln = NULL;