add warning when DirCache is 0 and DirPort is enabled. Also force enable...

I have changed DirCache so that if DirCache is disabled and DirPort is not then DirCache will be overridden and set to 1. I have added a note about this in the man page.

Issue #40718 (closed) requests that DirPortFrontPage (which requires DirPort) be usable when DirCache is disabled. As this does not appear possible without significant changes I thought setting DirCache automatically was a fair compromise. Also I don't know of a reason you would want to have DirCache on it's own, without having DirPort, bridge relay or something similar enabled, so it seemed sensible to me to have DirPort override DirCache. We could possibly also have BridgeRelay also overwrite DirCache and other options that require DirCache.

Fixes #40718 (closed)