Log Message:
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