Always call _arena_set_foe() for arena monsters whose target is (0, 0).

Obviates the need for setting the monster's target via an ME_ALERT event in mons_place().

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8538 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on January 18, 2009
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