shill: Splits netlink_message and generic_... out of nl80211_message.*

This CL just splits netlink_message.h/.cc and
generic_netlink_message.h/.cc out of nl80211_message.h/.cc.  The name
"generic" is as regrettable as it is accurate -- messages in those files
use the |genlmsghdr| which is the "generic netlink message header".  This
change necessitated modifiying header file includes.  Other than those
minor (required) changes, this CL represents a straight cut (from
nl80211_message.*) and paste.

BUG=None
TEST=unittest

Change-Id: I79d1f96d8e942c1ca602015f48ce74099899fe63
Reviewed-on: https://gerrit.chromium.org/gerrit/47859
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Wade Guthrie <wdg@chromium.org>
Reviewed-by: Wade Guthrie <wdg@chromium.org>
Tested-by: Wade Guthrie <wdg@chromium.org>
diff --git a/config80211.h b/config80211.h
index 9582f11..cfd92b5 100644
--- a/config80211.h
+++ b/config80211.h
@@ -70,7 +70,7 @@
 #include <base/bind.h>
 #include <base/lazy_instance.h>
 
-#include "shill/nl80211_message.h"
+#include "shill/netlink_message.h"
 
 struct nlmsghdr;
 
@@ -80,7 +80,6 @@
 class EventDispatcher;
 struct InputData;
 class IOHandler;
-class NetlinkMessage;
 class NetlinkSocket;
 
 // Config80211 is a singleton that coordinates sending netlink messages to,
@@ -100,8 +99,7 @@
   typedef base::Callback<void(const NetlinkMessage &)> NetlinkMessageHandler;
 
   // Encapsulates all the different things we know about a specific message
-  // type like its name, its id, and, eventually, a factory for creating
-  // messages of the designated type.
+  // type like its name, and its id.
   struct MessageType {
     MessageType();