networkd: reorder gperf fields

networkd: reorder gperf fields

In eb64b435ebb2a7cb1274d5 ActiveSlave/PrimarySlave were inserted in between
IPv6AcceptRA and IPv6AcceptRouterAdvertisements and the comment then didn't
make sense. It turns out that gperf does not understand C-style comments, and
that there's no comment syntax in the keywords section. The following was
generated:

      {"/* legacy alias for the above */"},
      {"Network.IPv6AcceptRA",                   config_parse_tristate,                          0,                             offsetof(Network, ipv6_accept_ra)},

In practice this wouldn't lead to problems because this fake pattern would be
hard to match, but it seems better to remove the comments altogether.

Readers of the .gperf file will have to look for the repeated output field to
notice legacy options. To make this easier, let's always keep the legacy option
second.