Support OpenSSL 3.0
PPP, L2TP: fix PPPOL2TP_AUTH_SUPPORT == 0 support
PPP, PPPoS: fix memory leak when disconnecting if there are remaining input bytes
PPP: fix build warning on wrong cast from void* to unsigned long
PPP: clear address state before clearing address
PPP: remove unused and confusing return values other than ERR_OK for ppp_connect and ppp_listen
PPP: use pbuf_coalesce() instead of private ppp_singlebuf()
PPP: remove ppp_singlebuf