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/fatal-signal.c

@@ -28,10 +28,10 @@
 #include "shash.h"
 #include "socket-util.h"
 #include "util.h"
-
-#define THIS_MODULE VLM_fatal_signal
 #include "vlog.h"
 
+VLOG_DEFINE_THIS_MODULE(fatal_signal)
+
 /* Signals to catch. */
 static const int fatal_signals[] = { SIGTERM, SIGINT, SIGHUP, SIGALRM };