network: Update rtsol options

- Use "-6" instead of "--noipv4" as it seems more appropriate based on
  the dhcpcd(8) manual page.
- Remove "-f /dev/null" as it seems unnecessary with "-6".
- Remove "--persistent" as it is in the default /etc/dhcpcd.conf,
  and this way the user can change it, if they would like to (either
  through dhcpcd_flags or by editing /etc/dhcpcd.conf).

The "-b" (or "--background") option is needed to avoid a timeout error
message (and a delay in booting), so it is still left in place.

etc/rc.d/network

@@ -1,6 +1,6 @@
 #!/bin/sh
 #
-# $NetBSD: network,v 1.84 2020/10/15 07:05:27 kim Exp $
+# $NetBSD: network,v 1.85 2020/10/16 15:31:04 kim Exp $
 #
 
 # PROVIDE: network
@@ -325,9 +325,7 @@ network_start_interfaces()
 				;;
 			rtsol)
 				if ! checkyesno dhcpcd; then
-					/sbin/dhcpcd -n -f /dev/null \
-						--background --persistent \
-						--noipv4 \
+					/sbin/dhcpcd -n6b \
 						${dhcpcd_flags} $int
 				fi
 				;;