Bring dhcpcd support up to date (FS#66349)

With dhcpcd 9, the rundir has moved. We follow this change and no longer
support older versions of dhcpcd.
While here, also support information only use on IPv6, which has been
possible with dhcpcd for quite a while already.

Reported by: Antonio Rojas <arojas@archlinux.org>

src/lib/dhcp/dhcpcd

@@ -7,10 +7,7 @@ dhcpcd_start() {
       6) options=$DhcpcdOptions6;;
       *) return 1;;
     esac
-    if [[ $2 == "noaddr" ]]; then
-        report_error "Using 'dhcpcd' for configuration without address assignment is unsupported"
-        return 1
-    fi
+    [[ $2 == "noaddr" ]] && options+=" --inform6"
     # If using own dns, tell dhcpcd to NOT replace resolv.conf
     [[ $DNS ]] && options+=" -C resolv.conf"
     if ! do_debug do_readable dhcpcd "-$1" -q -t "${TimeoutDHCP:-30}" $options "$Interface"; then
@@ -21,7 +18,7 @@ dhcpcd_start() {
 
 dhcpcd_stop() {
     local stop="-x"
-    if [[ -f "/run/dhcpcd-$Interface-$1.pid" ]]; then
+    if [[ -f "/run/dhcpcd/$Interface-$1.pid" ]]; then
         is_yes "${DHCPReleaseOnStop:-no}" && stop="-k"
         do_debug dhcpcd "-$1" -q $stop "$Interface" > /dev/null
     fi