Add sequence numbers to game state

This oopses on output dependency violations, e.g. two threads doing a
read-modify-write without synchronization, or the one thread nesting
several read-modify-writes.  Such bugs are difficult to spot, and tend
to be abusable.  I figure we have quite a few of them.

New struct emptypedstr member seqno.  Make sure all members of unit
empobj_storage share it.  Initialize it in files: main() and
file_sct_init().  Set it in ef_blank() and new ef_set_uid() by calling
new get_seqno().  Use ef_set_uid() when copying objects: swaps(),
doland(), doship(), doplane(), dounit(), delete_old_news().  Step it
in ef_write() by calling new new_seqno().

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

include/loan.h

@@ -44,6 +44,7 @@ struct lonstr {
     /* initial part must match struct empobj */
     short ef_type;
     short l_uid;
+    unsigned l_seqno;
     time_t l_timestamp;
     /* end of part matching struct empobj */
     natid l_loner;		/* loan shark */