Use CLOCK_MONOTONIC in dhcp6c

Rather than restarting rdnssd (and indirectly dhcp6c) to prevent the loss of IPv6 after rebooting the router (#604 #613 #655), using a monotonic clock source seems to be a more proper solution.

Asus has changed radvd code to use a monotonic clock source as well (see radvd/timer.c on which I have based my changes, although the changes in upstream radvd seems to differ) and has commented out the restarting of the services in ntp.c.

Without this change below and without restarting dhcp6c, I lost IPv6 some time after a reboot.