fix bz16 - book becoming cursed while being read

fix bz16 - book becoming cursed while being read

Attempting to read a cursed spellbook fails with a nasty effect.  But
a non-cursed book can become cursed while being read (malignant aura
after Wizard has been killed).  Assuming no interruption for other
reasons, the read would finish, the spell be learned, and then the
nasty effect would be given.  This changes things so that if the book
being read becomes cursed and the hero notices (book's bknown flag is
set), the read-in-progress will be interrupted.  Resuming will take
the attempting-to-read-a-cursed-book path.  Unfortunately, if the
hero doesn't notice, the old behavior still applies.  Maybe the new
behavior should happen even if bknown isn't set (but then player
won't be told why the interruption occurred).