Add option to pg_ctl to choose event source for logging

pg_ctl will log to the Windows event log when it is running as a service,
which is the primary way of running PostgreSQL on Windows. This option
makes it possible to specify which event source to use for this, in order
to separate different instances. The server logging itself is still controlled
by the regular logging parameters, including a separate setting for the event
source. The parameter to pg_ctl only controlls the logging from pg_ctl itself.

MauMau, review in many iterations by Amit Kapila and me.

src/bin/pgevent/pgevent.c

@@ -26,7 +26,7 @@ HANDLE		g_module = NULL;	/* hModule of DLL */
  * The maximum length of a registry key is 255 characters.
  * http://msdn.microsoft.com/en-us/library/ms724872(v=vs.85).aspx
  */
-char		event_source[256] = "PostgreSQL";
+char		event_source[256] = DEFAULT_EVENT_SOURCE;
 
 /* Prototypes */
 HRESULT		DllInstall(BOOL bInstall, LPCWSTR pszCmdLine);