improve robustness of ppp readiness notification

there was a race where ip-up could write ifname and then
ip6-up could write its outputs and then test ifname and
signal ready before ip-up had written the rest of its outputs

modules/ppp/common.nix

@@ -40,15 +40,17 @@ let
     set +o nounset
     if test -n "''${DNS1}" ;then echo ''${DNS1} > ns1 ; fi
     if test -n "''${DNS2}" ;then echo ''${DNS2} > ns2 ; fi
-    test -e ipv6-address && echo >/proc/self/fd/10
+    touch ip-up
+    test -e ipv6-up && echo >/proc/self/fd/10
   '';
   ip6-up = writeAshScript "ip6-up" { } ''
     exec >&5 2>&5
     . ${serviceFns} 
     in_outputs ${name}
-    echo $4 > ipv6-address
     echo $5 > ipv6-peer-address
-    test -e ifname && echo >/proc/self/fd/10
+    echo $4 > ipv6-address
+    touch ipv6-up
+    test -e ip-up && echo >/proc/self/fd/10
   '';
   literal_or_output =
     let