Set the secs field in the BOOTP header.

We mostly follow RFC 2131, which says that secs is the number of
seconds "since client began address acquisition or renewal
process", and thus set secs to zero on renew. This is different
from our current behaviour, which keeps on counting without
resetting secs to zero on renew.

Bug: 19704592
Change-Id: Ifbb7644094c579be626ffb698eee87047425dbf0