add standard-based PPP framing (RFC1661, RFC1662) as reference points

add standard-based PPP framing (RFC1661, RFC1662) as reference points

- PPP_ENCAP_RFC1661: Plain PPP. “Synchronous” in the ’90s-era language,
  because the start and end of the frame are known from external means.

  In modern instances, this is because they arrive in a complete packet with
  a known length from its lower-level encapsulation. (TLS or DTLS records in
  our expected use cases.)

  This is what `pppd sync` does.

- PPP_ENCAP_RFC1662: “PPP in HDLC-like framing.”

  This is what `pppd` without `sync` does.

At some point, we will be able to add automated testing of our PPP
implementation's ability to communicate with `pppd`.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>