Give savefiles an identifting header. SAve format version gets bumped.

advent.h

@@ -233,7 +233,12 @@ typedef struct {
  * files afterwards.  Otherwise you will get a spurious failure due to the old version
  * having been generated into a check file.
  */
-#define SAVE_VERSION	29
+#define SAVE_VERSION	30
+
+/*
+ * Goes at start of gile so sabes can be identified by file(1) and the like.
+ */
+#define ADVENT_MAGIC	"open-adventure\n"
 
 /*
  * If you change the first three members, the resume function may not properly
@@ -241,8 +246,7 @@ typedef struct {
  * when you do that.
  */
 struct save_t {
-    int64_t savetime;
-    int32_t mode;		/* not used, must be present for version detection */
+    char magic[sizeof(ADVENT_MAGIC)];
     int32_t version;
     struct game_t game;
 };

saveresume.c

@@ -25,8 +25,7 @@ struct save_t save;
 int savefile(FILE *fp, int32_t version)
 /* Save game to file. No input or output from user. */
 {
-    save.savetime = time(NULL);
-    save.mode = -1;
+    memcpy(&save.magic, ADVENT_MAGIC, sizeof(ADVENT_MAGIC));
     save.version = (version == 0) ? SAVE_VERSION : version;
 
     save.game = game;

tests/resumefail2.chk

@@ -11,7 +11,7 @@ down a gully.
 Can't open file y, try again.
 
 I'm sorry, but that Adventure was begun using Version -133.-7 of the
-save file format, and this program uses Version 2.9.  You must find an instance
+save file format, and this program uses Version 3.0.  You must find an instance
 using that other version in order to resume that Adventure.
 
 You're in front of building.