Halo-related cleanups: limit the halo's radius to (roughly) FOV, add

sanity checks to make sure we're not looking for monsters outside the monster grid, and reduce some code duplication.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@3671 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on March 16, 2008
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