clear_status_flags_on_sybil might want to clear more flags

clear_status_flags_on_sybil contains a comment saying "it's easy to add a new flag but forget to add it to this clause."

It looks like we may have forgot the following flags:

• is_hs_dir
• version_known?
• version_supports_extend2_cells?
• has_bandwidth
• has_exitsummary?
• bw_is_unmeasured? (set to 1?)
• bandwidth_kb
• has_guardfraction
• guardfraction_percentage

To deal with the root cause, should we instead zero out the entire routerstatus_t, then copy the fields we need back in? (This would zero new fields on sybils by default.)

We could also implement a unit test for clear_status_flags_on_sybil that checks that certain (important?) flags are cleared, or that all flags are cleared (?).