Add Elf and Vaults into DEPTHs of some ice cave entries, in order to generate them earlier in a game where the player doesn't descend to at least D:20 before going to those branches. Use Elf:2-6 to be kind to those who check out Elf:1 early and possibly back away because of the arrival vault.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@10556 c06c8d41-db1a-0410-9941-cceddc491573

Created by  evktalo  on August 17, 2009
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