vlog: Introduce VLOG_DEFINE_THIS_MODULE for declaring vlog module in use.

Adding a macro to define the vlog module in use adds a level of
indirection, which makes it easier to change how the vlog module must be
defined.  A followup commit needs to do that, so getting these widespread
changes out of the way first should make that commit easier to review.

lib/netdev-linux.c

@@ -61,9 +61,9 @@
 #include "socket-util.h"
 #include "shash.h"
 #include "svec.h"
-
-#define THIS_MODULE VLM_netdev_linux
 #include "vlog.h"
+
+VLOG_DEFINE_THIS_MODULE(netdev_linux)
 
 /* These were introduced in Linux 2.6.14, so they might be missing if we have
  * old headers. */