dhcpcd: Wait for carrier before active

In order to ensure that network interfaces are active, but not
necessarily fully configured, before *network.target* is reached, the
*dhcpcd.service* unit now waits until at least one interface has
processed the `CARRIER` event before being marked as active.  This is
accomplished using the *sd_notify* infrastructure, via the
`systemd-notify` command, executed in a client configuration hook
script.

package/dhcpcd/dhcpcd.service

@@ -4,7 +4,8 @@ Wants=network.target
 Before=network.target
 
 [Service]
-ExecStart=/usr/sbin/dhcpcd -q --nobackground
+Type=notify
+ExecStart=/usr/sbin/dhcpcd -q --nobackground --env NOTIFY_SOCKET=${NOTIFY_SOCKET}
 
 [Install]
 WantedBy=multi-user.target