Tidy up slays

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The resurrection of ​http://angband.oook.cz/forum/showthread.php?t=5371 has caused me to find some nasty bugs in the v4 slay code - which will need fixing independent of the archery and info stuff.

1. There is cruft in attack.c because mult is no longer needed there - it should be dealt with in calc_damage.

2. But there's a problem, because the slay_mult array doesn't add a % for the HURT_ flags. I think I originally thought this would be done outside the slay code, back in attack.c, but it can't, because the whole point is that calc_damage returns the total damage, it can't then be broken down into chunks for a HURT_ increment to be applied to base_dam.

3. object_slay_mults sets the mult array value to -1 if the HURT_ flag is present - but this is immediately reset to 0 because 0 > -1!

I think the solution is to set the mult to 100 for these, but needs careful checking.

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[ea27dff] and the two preceding commits.