Make local includes consistent; use double quotes for local includes

except for generated ones where we use angle brackets.
Also g/c unused compat.h.

plugins/sample/sample_plugin.c

@@ -49,9 +49,9 @@
 #include <pwd.h>
 #include <stdarg.h>
 
-#include <sudo_plugin.h>
-#include <missing.h>
 #include <pathnames.h>
+#include "sudo_plugin.h"
+#include "missing.h"
 
 /*
  * Sample plugin module that allows any user who knows the password