rp-pppoe plugin: Import some fixes from rp-pppoe-3.10

* Make timeout for PADO/PADS be the total time we wait for, not the
  time that each individual select call waits for.

* Open the discovery socket in PPPOEConnectDevice() rather than
  discovery().

* Open the session socket earlier in PPPOEConnectDevice() to avoid
  missing session packets sent by the server just after the PADS
  packet.

Signed-off-by: Paul Mackerras <paulus@samba.org>