ppp: write ifindex as output

because what happens if the service is restarted but the new ppp0 is
a different interface than the old one so that services which had
bound to it with the old name are now not getting new data

(I am not 100% that this actually happens but it seems like it would
be good to avoid it if it does)

modules/ppp/common.nix

@@ -36,6 +36,7 @@ let
     echo $3 > speed
     echo $4 > address
     echo $5 > peer-address
+    cat /sys/class/net/$1/ifindex > ifindex
     set +o nounset
     if test -n "''${DNS1}" ;then echo ''${DNS1} > ns1 ; fi
     if test -n "''${DNS2}" ;then echo ''${DNS2} > ns2 ; fi