amazon s3 index file improvements

amazon s3 index file improvements

Turns out duplicate index files do not need to be stored when usedirs is in
use, just when it's not. Ikiwiki is quite consistent about using page/ when
usedirs is in use. (The only exception is the search plugin, which needs
fixing.)

This also includes significant code cleanup, removal of a incorrect special
case for empty files, and addition of a workaround for a bug in the amazon
perl module.