Add option to exclude dropped items from autopick

This is based on a feature in UnNetHack (and I think some other variants
as well).  If the hero intentionally drops an item with 'pickup_dropped'
disabled, don't autopick it back up when walking over that square again.

Typically when the player drops an item, it's because she doesn't want
it in her inventory any more, and this option stops autopickup from
defeating that goal (especially useful for tasks like stash management
without a container).  Players have come up with workarounds to this
problem like toggling autopickup when approaching their stash pile or
adding name-based autopickup exceptions to allow them to exclude
individual items from autopickup, but this behavior should reduce the
need for those things.

I think 'pickup_dropped' is a little unfortunate because it suggests
equivalence to 'pickup_thrown' (i.e. any dropped items will be
automatically picked up regardless of autopickup exceptions).  Calling
it something like 'nopick_dropped' might be better, but as far as I can
tell options cannot start with the word 'no' because it's interpreted as
a negation of the rest of the option name.

dat/opthelp

@@ -50,6 +50,7 @@ null           allow nulls to be sent to your terminal             [True]
                delay code) if moving objects seem to teleport across rooms
 perm_invent    keep inventory in a permanent window               [False]
 pickup_thrown  override pickup_types for thrown objects            [True]
+pickup_dropped evaluate pickup_types for dropped objects           [True]
 pushweapon     when wielding a new weapon, put your previously    [False]
                wielded weapon into the secondary weapon slot
 quick_farsight usually skip the chance to browse the map when     [False]