PPPoE MTU is not correctly set from values on interfaces.php
Setting the MTU on interfaces.php is ignored, and overridden by what's configured on the PPP instead. What's set by interfaces.php should be used, and should set what's set on the PPP, rather than ignored.
Fixes #2209. Obey the mtu value set on the interfaces.php page. Though this value will be overwritten if there is a configuration under PPP settings tab. Maybe a good idea is to set MRU at the same value if not set?