libsystemd-dhcp: DNS name option must be a multiple of 4 bytes
libsystemd-dhcp: Rename function to be clearer that options are parsed
libsystemd-dhcp: Compute UDP checksum only if set

A checksum field with value zero means no UDP checksum has been
computed for the packet.
libsystemd-dhcp: Fix stopping of DHCP client

Go back to Init state independent of the current state the client
is in.
libsystemd-dhcp: Update secs field only when sending Discover

Compute the 'secs' field as seconds since start of lease acquisition
procedure. Start off with a value of zero and increase it only for
each resent DHCP discover message. See the discussion before and
after http://www.ietf.org/mail-archive/web/dhcwg/current/msg05836.html
and Section 3.1 of RFC 2131.