add support for PPP-based protocols

add support for PPP-based protocols

This started out as the 'f5' branch, which was then rebased (by DL) onto a
more recent master as 'ppp_rebased'.

This was then squashed down into a single commit, with all of the bits
depending on "real" PPP-based VPN protocols removed, so that only the shell
of generic PPP support remains, including PPP protocol negotation (LCP,
IPCP, IP6CP) and mainloop.

Almost all of this code was done by David Woodhouse and Daniel Lenski in
May 2020, with a few more recent bugfixes by DL in early February 2021.

The remaining shell supports two different encapsulations of PPP:

    - 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.

The following commits will add tests to demonstrate the functionality of
OpenConnect's PPP implementation at the level of this generic encapsulation.

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