Add sequence numbers to game state (experimental)

Add sequence numbers to game state (experimental)

This catches output dependency violations, e.g. two threads doing a
read-modify-write without synchronization.

New struct emptypedstr member seqno.  Make sure all members of unit
empobj_storage share it.  Set it in ef_blank() and ef_set_uid(), step
it in ef_write().  fairland and files don't use ef_set_uid(); need to
set it manually in files.c's main() and file_sct_init().

Factor do_read() out of fillcache() to make it available for
new get_seqno().