Add num_digits parameter to getnumber

In some integer prompts, it doesn't make sense to enter a huge integer. It would probably be better to disallow typing huge integers in in the first place rather than only relying on clamping them after the fact.

Example:

1. Quest->Misc Data->Triforce Pieces
2. Click any of the 8 squares within the triforce fragments
3. Type a very large number in the "Piece Number" prompt
4. Click 'OK'
5. Observe that fragment number has been clamped to 8

This is only a suggestion to add a parameter to the getnumber function in zquest.cpp and not a suggestion to remove the clamping.