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/poll-loop.c

@@ -28,10 +28,10 @@
 #include "fatal-signal.h"
 #include "list.h"
 #include "timeval.h"
-
-#define THIS_MODULE VLM_poll_loop
 #include "vlog.h"
 
+VLOG_DEFINE_THIS_MODULE(poll_loop)
+
 /* An event that will wake the following call to poll_block(). */
 struct poll_waiter {
     /* Set when the waiter is created. */