network: disable IPv6AcceptRA= in several default config

Also,
- drop DHCP=no, as it is the default setting,
- enable IPv6SendRA= for wifi access point,
- enable MulticastDNS= for wifi adhoc mode.

network/80-wifi-ap.network.example

@@ -17,3 +17,5 @@ WLANInterfaceType=ap
 Address=0.0.0.0/24
 DHCPServer=yes
 IPMasquerade=both
+IPv6AcceptRA=no
+IPv6SendRA=yes