ospfd: use random() to pick initial DD seq#

ospfd: use random() to pick initial DD seq#

While the idea for this came the other way around - removing
quagga_time() - using random() is actually a better idea here.  It's
seeded by time to begin with, but if ospfd restarts several times in a
short timespan it won't run straight into the same sequence number.

(Should also update the random seed to include microseconds so restarts
within a second use a different seq#)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>