Docs: improvements on upgrade docs
Fix group permissions for REMOTE_PODIRECTORY

also be a bit more careful about these operations in case REMOTE_PODIRECTORY
were not set, you don't want `rm -rf /*`!
Fix typo in MySQL-python rev.spec.
Add public cache control for proxies

This change (also for bug 2935) allows proxies to do caching, as well as
browsers.