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/files.c

@@ -136,7 +136,7 @@ main(int argc, char *argv[])
     for (i = 0; i < EF_MAX; i++) {
 	if (!EF_IS_GAME_STATE(i))
 	    continue;
-	if (!ef_open(i, EFF_CREATE | EFF_NOTIME, -1)) {
+	if (!ef_open(i, EFF_CREATE | EFF_NOTIME)) {
 	    perror("ef_open");
 	    exit(1);
 	}