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/daemon.c

@@ -32,10 +32,10 @@
 #include "socket-util.h"
 #include "timeval.h"
 #include "util.h"
-
-#define THIS_MODULE VLM_daemon
 #include "vlog.h"
 
+VLOG_DEFINE_THIS_MODULE(daemon)
+
 /* Should we run in the background? */
 static bool detach;