Improve PPP netif server support (IDFGH-14720)

[gonzzor]

Description

The previous introduction of server support was a nice addition, improve this by making it easier to run both a PPP server and PPP client on different interfaces.

Note that if CONFIG_LWIP_PPP_SERVER_SUPPORT=y and ppp_passive isn't set the code will call ppp_connect() instead of ppp_listen(). However, ppp_connect() and ppp_listen() are the same for pppos so it shouldn't matter.

For PPPoE ppp_listen() isn't really supported.

Related

Testing

Tested with PPP over serial as server and esp-modem for uplink.

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