pppoe: T5630: allow to specify MRU in addition to already configurable MTU

Set the MRU (Maximum Receive Unit) value to n. PPPd will ask the peer to send
packets of no more than n bytes. The value of n must be between 128 and 16384,
the default was always 1492 to match PPPoE MTU.

A value of 296 works well on very slow links (40 bytes for TCP/IP header + 256
bytes of data). Note that for the IPv6 protocol, the MRU must be at least 1280.

CLI:

set interfaces pppoe pppoe0 mru 1280

(cherry picked from commit e062a8c11856f213983f5b41f50d4f9dbc0dde0f)