Large monster can knock back smaller ones

When a monster at least two sizes larger hits another one,
there's a chance the smaller defender will be knocked back.

This applies also to hero, attacking when polymorphed to
a large monster, or defending from a large monster.

Most of the monsters that can knock back are giants and dragons.

Idea and some of the code from EvilHack.

doc/fixes3-7-0.txt

@@ -971,6 +971,7 @@ wielding a bec de corbin makes ravens generate peaceful
 moving with 'm' prefix allows hero to enter a known pit carefully
 rangers always succeed in disarming bear traps, unless impaired
 bigroom variant 2 may have ice floor in unlit areas
+some large monsters can knock back smaller monsters with a hit
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository