NumDirectoryGuards should track NumEntryGuards

If you change NumEntryGuards to 1, your Tor will still contact 3 relays for directory information, since you didn't realize you had to set NumDirectoryGuards too. This fails the 'no surprises' principle.

At the same time, we have this check:

  if (options->UseEntryGuards && ! options->NumEntryGuards)
    REJECT("Cannot enable UseEntryGuards with NumEntryGuards set to 0");

but no corresponding check for "UseEntryGuardsAsDirGuards 1 but NumDirectoryGuards 0".

I think the right behavior should be to treat NumDirectoryGuards 0 as "use the value from numentryguards", and change that to the default.

Bug encountered by Isis.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information