The initial sequence number for RIP md5 authentication was always zero.
Bad as when bird e.g. was running for two weeks and then restarted it
would take another two weeks until the peers of this router would accept
data again from this router, as the sequence number would be too low.
Changed to use the the current system time as the starting sequence
number which is a more sane start value.
(by Andreas, cleaned up by me)