Don't allow monsters wielding two-handed weapons to pick up shields.

Partial fix for [2508934]; the bug is still there because monster nagas count as SIZE_BIG, not SIZE_LARGE. I'll let dolorous fix the monster flags. :)

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8926 c06c8d41-db1a-0410-9941-cceddc491573

Created by  haranp  on February 6, 2009
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