Fix potions of decay created by mummy curse sometimes being inscribed.

Change inscription prompt to work as intended by David, after all. :) Make manuals use a reading counter (plus2) that is initialized with 3 + random2(15). Once it reaches 0, the manual crumbles to dust.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6443 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on July 7, 2008
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