Kernel+ifconfig: Add an MTU value to NetworkAdapter

This defaults to 1500 for all adapters, but LoopbackAdapter increases
it to 65536 on construction.

If an IPv4 packet is larger than the MTU, we'll need to break it into
smaller fragments before transmitting it. This part is a FIXME. :^)

Kernel/FileSystem/ProcFS.cpp

@@ -345,6 +345,7 @@ Optional<KBuffer> procfs$net_adapters(InodeIdentifier)
         obj.add("packets_out", adapter.packets_out());
         obj.add("bytes_out", adapter.bytes_out());
         obj.add("link_up", adapter.link_up());
+        obj.add("mtu", adapter.mtu());
     });
     array.finish();
     return builder.build();