Artifact and #offer rebalance, part 1: sacrifice gifts

In 3.6, artifact gifts are often either a) entirely useless or
b) gamebreaking, neither of which is really ideal from a balance
perspective.

This commit aims to make artifact gifts more useful in the early
game by greatly increasing the chance for situational artifacts to
generate positively enchanted.  However, the most powerful
artifacts will now only be gifted if you offer a high-value corpse,
meaning that they are only likely to be accessible later in the
game.  The selection of which artifact to gift has become more
complicated in order to a) increase the chance that it fits the
character and b) reduce cheese strategies (e.g. it is no longer
possible for elves to force the gifting of Stormbringer as the
first sacrifice gift).

include/artifact.h

@@ -49,6 +49,8 @@ struct artifact {
     aligntyp alignment; /* alignment of bequeathing gods */
     short role;         /* character role associated with */
     short race;         /* character race associated with */
+    schar gen_spe;      /* bias to spe when gifted or randomly generated */
+    uchar gift_value;   /* minimum sacrifice value to be gifted this */
     long cost;          /* price when sold to hero (default 100 x base cost) */
     char acolor;        /* color to use if artifact 'glows' */
 };