Store sequence numbers more compactly

Store them and ef_type in bit-fields.

Allocate eight bits for ef_type.  Values range from 0 to EF_GAME
(currently 16), so this is plenty.

Allocate twelve bits for sequence numbers.  Sequence number mismatches
are now missed when the numbers are equal modulo 2^12.  Still
sufficiently improbable.

Common machines store the bit-fields in a 32 bit word.  There are
twelve bits left in that word for future use.  Total savings 16 bits,
which is exactly what the previous commit spent on wider uids on
common machines.

include/game.h

@@ -38,9 +38,9 @@
 
 struct gamestr {
     /* initial part must match struct empobj */
-    short ef_type;
+    signed ef_type: 8;
+    unsigned game_seqno: 12;
     int game_uid;
-    unsigned game_seqno;
     time_t game_timestamp;
     /* end of part matching struct empobj */
     char game_upd_disable;	/* updates disabled? */