More stuff

- Fixes more of 671 by checking the user directory and loading a file there if found; this allows the user to set up a custom pref.prf file that loads in addition to the one in lib/pref.
- Fixes 1628 by allowing the description to be toggled with ?. Default state is off.
- Fixes 1776 and adds some functions to set the valid/enable state of dynamic menus.
- Fixes 1780 by forcing all drop flags on first kill and displaying only what is used by the game (lore->drop_item and lore->drop_gold are ignored now).
- Fixes issue with squelch type name (http://angband.oook.cz/forum/showpost.php?p=85439&postcount=251).