getobj() magic marker candidate items

getobj() magic marker candidate items

When applying a magic marker, only list known blank scrolls and known
blank spellbooks as likely candidates to write on.  Accepts any scroll
or book (but non-blank ones will get rejected by the writing code).
Attempting to choose some other class of item yields "that is a silly
thing to write on", same as before.

This was requested during beta testing and I'd swear that I checked it
in a long time ago, but it wasn't here.