Clean up how game state file sizes are checked

New struct empfile member nent replaces ef_open() parameter nelt.
Cleaner, because the expected size is a property of the file, not of
how it's used.  Also fixes empdump to check file sizes.

Complication: with EFF_CREATE, ef_open() creates an empty file, to be
extended to the correct size.  Callers passed nelt argument -1 along
with EFF_CREATE, to make ef_open() accept the empty file.  Can't do
the same for empfile member nent.  Instead, make ef_open() not check
the (zero) size then.

Replaces commit 5750107b, v4.3.15.

src/util/fairland.c

@@ -261,7 +261,7 @@ main(int argc, char *argv[])
 	fprintf(stderr, "Can't chdir to %s (%s)\n", gamedir, strerror(errno));
 	exit(EXIT_FAILURE);
     }
-    if (!ef_open(EF_SECTOR, EFF_MEM | EFF_NOTIME, WORLD_SZ())) {
+    if (!ef_open(EF_SECTOR, EFF_MEM | EFF_NOTIME)) {
 	perror("ef_open");
 	exit(1);
     }