Miscellaneous cleanup to silence compiler warnings seen on Mingw.

Remove some dead code, conditionally declare some items or call
some code, and fix one or two declarations.

src/bin/pgevent/pgevent.c

@@ -42,7 +42,6 @@ HRESULT
 DllInstall(BOOL bInstall,
 		   LPCWSTR pszCmdLine)
 {
-	size_t		ret;
 
 	if (pszCmdLine && *pszCmdLine != '\0')
 		wcstombs(event_source, pszCmdLine, sizeof(event_source));