Make git-daemon-export-ok exclusion a config value

Just enabling this would be an automatic change, and many people
would suddenly see all their projects disappear and not know what
happened. I like the idea of respecting git-daemon-export-ok but I
can't reasonably expect everyone to go through all their repositories
and add this when they upgrade, so the exclusion is set by a config
option that's off by default.

Also, Gitweb doesn't do this at all so the odds of people having this
magic file set are even lower.