Rhizome manifest parser now parses and validates all known fields, informs
about unsupported fields, and unpacks fields into relevant struct manifest
elements where appropriate. Is also stricter about whitespace.
Rhizome fetch code now logs debug messages if DEBUG_RHIZOME_RX bit is on.
Was not transmitting actual HTTP server port in rhizome announcements, was
always transmitting port 4110.
When trying for a free HTTP server port, sometimes bind() succeeds but listen()
fails with EADDRINUSE, so new logic to deal with that.
