Disable guard nodes with Tor2web mode

On #6866 (moved), rransom says:

Perhaps [Tor2web mode] should prevent users from operating hidden services, and turn off UseEntryGuards (as I understand it, this would disable the path-bias detection code). They're not getting anonymity anyway.

I think this is right. Guard nodes exist to prevent profiling attacks, and tor2web instances are not concerned about those.

Mike Perry has a patch as part of one of his #6866 (moved) branches. I'm giving this its own ticket number, though, so there can be some record of where we talked about this.