* Adds local file:// URL support to the [validator_list_sites] stanza.
The file:// URL must not contain a hostname. Allows a rippled node
operator to "sideload" a new list if their node is unable to reach
a validator list's web site before an old list expires. Lists
loaded from a file will be validated in the same way a downloaded
list is validated.
* Generalize file/dir "guards" from Config test so they can be reused
in other tests.
* Check for error when reading validators.txt. Saves some parsing and
checking of an empty string, and will give a more meaningful error.
* Completes RIPD-1674.
