Install named plugins into a separate directory

Installing named plugins into ${libdir} clutters the latter and is not
in line with common filesystem conventions.  Instead, install named
plugins into a separate directory, ${libdir}/named.

make/rules.in

@@ -28,6 +28,8 @@ localstatedir =	@localstatedir@
 mandir =	@mandir@
 datarootdir =   @datarootdir@
 
+plugindir =	${libdir}/named
+
 DESTDIR =
 
 @SET_MAKE@