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/mac-learning.c

@@ -29,10 +29,10 @@
 #include "tag.h"
 #include "timeval.h"
 #include "util.h"
-
-#define THIS_MODULE VLM_mac_learning
 #include "vlog.h"
 
+VLOG_DEFINE_THIS_MODULE(mac_learning)
+
 /* Returns the number of seconds since 'e' was last learned. */
 int
 mac_entry_age(const struct mac_entry *e)