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.

(cherry picked from commit c527b7fd5c)

bin/plugins/Makefile.in

@@ -59,14 +59,14 @@ clean distclean::
 	rm -f ${TARGETS} ${OBJS}
 
 installdirs:
-	$(SHELL) ${top_srcdir}/mkinstalldirs ${DESTDIR}${libdir}
+	$(SHELL) ${top_srcdir}/mkinstalldirs ${DESTDIR}${plugindir}
 	$(SHELL) ${top_srcdir}/mkinstalldirs ${DESTDIR}${mandir}/man8
 
 install:: filter-aaaa.@SO@ installdirs
 	${LIBTOOL_MODE_INSTALL} ${INSTALL_LIBRARY} filter-aaaa.@SO@ \
-		${DESTDIR}${libdir}
+		${DESTDIR}${plugindir}
 	${INSTALL_DATA} ${srcdir}/filter-aaaa.8 ${DESTDIR}${mandir}/man8
 
 uninstall::
-	${LIBTOOL_MODE_UNINSTALL} rm -f ${DESTDIR}${libdir}/filter-aaaa.@SO@
+	${LIBTOOL_MODE_UNINSTALL} rm -f ${DESTDIR}${plugindir}/filter-aaaa.@SO@
 	rm -f ${DESTDIR}${mandir}/man8/filter-aaaa.8

make/rules.in

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