Fix backspace key not working with counts

This fixes backspace so that it winds back player-provided counts for
queried keys (e.g. during #adjust), command inputs and item menus.

This bug also exists in upstream NitroHack.
Fix count overflow in query key prompt

This bug also exists in upstream NitroHack.
Show count when allowed in query key prompts

This shows player-entered counts during query key propmts, e.g. the
first prompt during #adjust, like in object selection menus.
Allow count correction at second prompt of #adjust

This allows the stack size of an object being #adjusted to be changed by
providing a count in the second letter prompt for the target letter.
This also means the count can be given when choosing 'adjust' from the
item action menu of inventory objects.