ID scrolls (and throne ID) ID 3-6 items regardless of BUC

This makes reading scrolls of identify while exploring much less worse
than stashing en masse then blessed-identifying everything, so players
know what things are earlier, meaning less carrying of junk around and
more variety in the early game.

As for thrones, this means players desperate for a full inventory ID
don't have to consider carrying an inventory full of unID'd items
halfway across the dungeon and back again.
Reduce to-hit bonus from +Luck to rnd(Luck)

Having an invisible attribute contribute more to to-hit than dexterity
was silly: dex 18 gives +4, max dex (with +5 gauntlets of dexterity)
gives +9 (most characters) or +11 (dwarves), while max luck gave +13
(which will now be +d13 instead).

This should make the game a bit less of a cakewalk for veterans, and put
newer players at less of a disadvantage for not knowing how luck works.
Learn weapon enchantment on hit based on skill/race

Unskilled/basic/skilled/expert has a 1 in 1000/200/100/50 chance to
discover the enchantment of weapons, launchers and ammunition.  Weapons
of the hero's race double this chance.

Technically the chance is slightly lower than this, since the code that
does this relies on the valid_weapon_hit flag in hmon_hitmon(), which is
only set if the weapon hit does more than 1 damage.

This also fixes a couple of places where the weapon object considered by
hmon_hitmon() may become a dangling pointer, for safety's sake.