DHCP: Support dhcpcd-9.x

This breaks <=dhcpcd-6.10.0 as they will lack the -P option. This should not be a problem as that's over 5 years old vs this new shiny.

Due to the way dhcpcd-9 uses privilege separation, when we kill the dhcpcd process waitpid(2) might return ECHILD so adjust code accordingly. When this happens, it has actually exited and this behaviour is noted by POSIX.

This fixes issue #410 (closed)