Clean up how game state file sizes are checked

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.