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().

include/sect.h

@@ -46,6 +46,7 @@ struct sctstr {
     /* initial part must match struct empobj */
     short ef_type;
     short sct_uid;		/* equals XYOFFSET(sct_x, sct_y) */
+    unsigned sct_seqno;
     time_t sct_timestamp;	/* Last time this sector was written to */
     natid sct_own;		/* owner's country num */
     coord sct_x;		/* x coord of sector */