netdev: use "relaxed" parsing

We parse each netdev file twice: once to determine the type and match conditions,
and then the second time properly. In bcde742e78
the flags for the first parsing were (inadvertently I assume) were changed to
emit warnings. But this first pass is called with only [Match] and [NetDev] sections,
so we'd get warnings about all other section types. The obvious solution would be
to remove CONFIG_PARSE_WARN again, but I think it's better to keep the warnings
and set CONFIG_PARSE_RELAXED: we do want to get warnings about malformed lines and
such, and _RELAXED is enough to kill warnings about unknown sections.

src/network/netdev/netdev.c

@@ -647,7 +647,7 @@ static int netdev_load_one(Manager *manager, const char *filename) {
         r = config_parse_many(filename, network_dirs, dropin_dirname,
                               "Match\0NetDev\0",
                               config_item_perf_lookup, network_netdev_gperf_lookup,
-                              CONFIG_PARSE_WARN, netdev_raw);
+                              CONFIG_PARSE_WARN|CONFIG_PARSE_RELAXED, netdev_raw);
         if (r < 0)
                 return r;