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).

doc/fixes3-7-0.txt

@@ -1486,6 +1486,10 @@ digging in ice was handled inconsistently, particularly if done at the span
 angry god may remove an intrinsic
 gelatinous cubes eat organic objects inside them
 pets with reflection were unwilling to attack floating eyes
+artifact gifts are rebalanced (easier to obtain; higher-value sacrifices are
+	needed for higher-value artifacts; lower-value artifacts are usually
+	gifted enchanted; unaligned artifacts are possible but rare even on
+	the first gift; artifacts you can't use well are less likely)
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository