Heavyshot: damage multiplier instead multishot

Some thrown or fired weapons now do 1dX damage where X is the skill
level:

 * crossbows
 * spears
 * javelins
 * shuriken
 * boomerangs

Heavyshot replaces multishot for weapons that had it, i.e. crossbows and
shuriken.

Spears now get a racial +1 skill for this calculation.

Javelins now get +1 skill for this calculation at basic skill or above,
so the bonus counts 1/2/3/4 instead of 1/1/2/3 for
unskilled/basic/skilled/expert.

This fixes a minor NetHack bug where monsters can multishot when stunned
even though the player cannot.