sd-dhcp6-client: Implement Elapsed Time option

sd-dhcp6-client: Implement Elapsed Time option

Implement Elapsed Time option as it is defined as MUST in RFC 3315,
section 22.9. The elapsed time value is a 1/100th of a second with
a max value of 0xffff, i.e. 655.35 seconds.

As the main loop might not be running yet when sd_dhcp6_client_start() is
called, fetch the monotonic time directly and not from the event loop
while in state DHCP6_STATE_STOPPED.