site: Rename setup_priority to our_name_later

This is also used for setting the transform direction - ie,
distinguishing us and them.  We are going to make the priority system
more subtle, and don't want to mess with that bit while changing the
priority rules.

No functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
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v2: Fix conflict due to rebase on top of bugfix
     "site.c: Cope with failure of transform `setkey' method."