Adjust Zin's donation routines to only take half your money at once, to

preserve the piety pool when it would cause an overflow, and to increase the maximum value such that donations won't be lost in a normal game.
I'm not sure what upper limit to set, especially considering that this is supposed to support DOS and possibly 16-bit values, so, seeing as durations are signed ints, I've set it to 32767, which you can only get all at once with a donation of a little less than 500000 gold.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8501 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on January 17, 2009
AK73N24BCYLHPTVYVO562AUM6KGLD5DUM572EPX43XYUUKBVOOXQC
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