Throwing nets again.

Added a number of functions that allow to differentiate between a net that currently traps a monster and one that doesn't, even if they are on the same square. This usually works but can fail if there's already a net lying on the target square.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@2021 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on August 20, 2007
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