gdhcp: set secs in a RFC 951- and 2131-compliant manner

gdhcp: set secs in a RFC 951- and 2131-compliant manner

This patch sets the BOOTP secs field in a RFC 951- and 2131-compliant
manner for DHCPv4 DISCOVER and REQUEST/SELECT packets.

Certain DHCP servers, such as that implemented in Mac OS X (< 10.7)
for its "Internet Sharing" feature, refuse to issue a DHCP lease to
clients that have not set a non-zero value in their DISCOVER or
REQUEST packets. In fact, based on http://hints.macworld.com/article.php?
story=20071223001432304, it's not non-zero but a value greater than four (4)
seconds to allow another "authoritative" DHCP server on the subnet to reply
first.

Side-by-side packet analysis of Mac OS X, iOS, Android, ISC and
Windows clients show that these clients set the BOOTP 'secs' field and
are successfully issued a DHCP lease by Mac OS X. By contrast, a
connman-based client will issue 10 back-to-back DISCOVER packets and
will not be returned a DHCP OFFER from the server.