Allow PPPoE MTU up to 1500

To be able to negotiate an MTU of 1500 over PPPoE, it is necessary that
the WAN MTU is set to at least 8 bytes more.
Fix compile error in PPPoE changes
Fix indentation
Do not set MTU to 1
Also take MRU value into account when configuring the MTU for the WAN interface
Merge pull request #558 from pinwing/master

Setting PPPoE MTU in web interface to 1500 did not work