Follow PPPoE plugin rename from ppp 2.5.0 (FS#79879)

The ppp project renamed their pppoe plugin to remove ambiguity with the
out-of-tree rp-pppoe plugin.

Reported by: yangling <327934575@qq.com>

src/lib/connections/pppoe

@@ -24,7 +24,7 @@ pppoe_up() {
 linkname $(quote_word "$Profile")
 ${PPPUnit:+unit $(quote_word "$PPPUnit")}
 updetach
-plugin rp-pppoe.so
+plugin pppoe.so
 nic-$Interface
 
 noauth