rp-pppoe 3.8: Updates

rp-pppoe 3.8: Updates
* Recommend ppp-oe, which is the PPPoE plugin module for PPP to
  enable the kernel mode PPPoE which is a lot faster, has lower
  latency and uses less CPU.
* Remove the module path from the configuration file since it's wrong
  and it's not needed.
* Don't check for the location of echo on the host, just use
  /bin/echo - it's only needed for the script that rp-pppoe installs.