Make some weaker artifact weapons more interesting (from Sporkhack)

All the listed weapons have a 20% instakill chance against their
respective target monsters, plus:
- Dragonbane is now a dwarvish spear that conveys fire, cold, shock, acid,
  poison, acid, and disintegration resistance while wielded.
- Giantslayer is now a sling that gives 18/** STR while wielded or
  in the alternate weapon slot.
- Werebane conveys polymorph control while wielded.
- Ogresmasher gives 25 CON while wielded.
- Trollsbane prevents trolls from leaving corpses while wielded.



git-svn-id: https://unnethack.svn.sourceforge.net/svnroot/unnethack/trunk@1441 f1e491ac-5ded-4967-80bf-9f4855be876a