Allow randomly generating node-ids instead of requiring the fingerprint.

To ease delopyment, "-genServerParams has changed".

 * "-genServerParams" is now a bool, and will by default generate a
   random node-id.
 * "-genServerParams -genServerParamsFP=<Base16 blob>" will convert the
   supplied bridge fingerprint to a node-id (the old behavior).

Either way of deriving node-id is belived to be secure.

 * https://lists.torproject.org/pipermail/tor-dev/2014-May/006929.html
 * https://lists.torproject.org/pipermail/tor-dev/2014-June/006936.html

The extra parameter was added because golang's flags library doesn't
support distinguishing between "set but used the default value" and
"not set, so you go the default value".