Load validator list from file:

* 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.

cfg/validators-example.txt

@@ -14,10 +14,8 @@
 #
 #   List of the validation public keys of nodes to always accept as validators.
 #
-#   The latest list of recommended validators can be obtained from
-#   https://ripple.com/ripple.txt
-#
-#   See also https://wiki.ripple.com/Ripple.txt
+#   Manually listing validator keys is not recommended for production networks.
+#   See validator_list_sites and validator_list_keys below.
 #
 #   Examples:
 #    n9KorY8QtTdRx7TVDpwnG9NvyxsDwHUKUEeDLY3AkiGncVaSXZi5
@@ -27,9 +25,13 @@
 #
 #   List of URIs serving lists of recommended validators.
 #
+#   The latest list of recommended validator sites can be
+#   obtained from https://ripple.com/ripple.txt
+#
 #   Examples:
 #    https://vl.ripple.com
 #    http://127.0.0.1:8000
+#    file:///etc/opt/ripple/vl.txt
 #
 # [validator_list_keys]
 #
@@ -39,6 +41,9 @@
 #   publisher key.
 #   Validator list keys should be hex-encoded.
 #
+#   The latest list of recommended validator keys can be
+#   obtained from https://ripple.com/ripple.txt
+#
 #   Examples:
 #    ed499d732bded01504a7407c224412ef550cc1ade638a4de4eb88af7c36cb8b282
 #    0202d3f36a801349f3be534e3f64cfa77dede6e1b6310a0b48f40f20f955cec945