Overhaul trapping net escape functions.

With the help of several factors (such as size, wielded weapon or stats) the game decides whether you should try to destroy the net to come free, or try to slip out of it. The same calculation also influences how long this will take.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@2278 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on October 1, 2007
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