Import dhcpcd-10.0.2 with the following changes:
* OpenBSD: add RTM_DESYNC to route(4) filter.
* Linux: fix disabling of kernel RA autoconf
* Linux: Improve learning IPv6 address flags
* Linux: risc-v fix vendor error
* Linux: consider IFF_LOWER_UP and !IFF_DORMANT for LINK_UP
* BSD: When we get RTM_NEWADDR the interface must have IFF_UP
* BSD: Fix non INET6 builds
* DHCP: Don't enforce the message came port 67
* privsep: Allow zero length messages through
* dhcpcd: deal with HANGUP and EPIPE better
* dhcpcd: Fix waitip address family
* privsep: Check if we have a root process before sending it stuff
* privsep: Only unlink control sockets if we created them
* common: Improve valid_domain and check correct return
* common: Allow hwaddr_ntoa to print an empty string
* privsep: Send only what we have put in the buffer to script env

(roy)