Issue reported by chadministratorwastaken: persistent inventory
window didn't update if reading a spellbook caused that book to fade
to blank paper.
Issue was reported for 'curses' but it was general. The core made
the possibly-revised book type become discovered but that wouldn't
update perm_invent unless the type of book hadn't been discovered.
So if plain spellbook wasn't known yet, the persistent inventory
window did update as intended, but if spellbook of blank paper was
already discovered, the window continued to show the old book type
instead of changing to reflect the revised type (until subsequent
activity triggered another perm_invent update).
Fixes#1246
c623ddf2e2