We actually allocated a pbuf chain only to iterate later the linked list
calling sio_write() for each pbuf, improved by calling sio_write() when
buffer is full and by recycling the pbuf, therefore only using one pbuf
for PPPoS output path.
Reworked pppos_write() and pppos_netif_output() to share more common
code into pppos_output_append() and pppos_output_last().
6a04357547