Fortinet: server rejects asyncmap and header compression options

Fortinet: server rejects asyncmap and header compression options

This appears to be a "feature" of all Fortinet servers, not just the one I have
access to. Openfortivpn calls pppd with the 'noaccomp nopfcomp default-asyncmap'
options: https://github.com/adrienverge/openfortivpn/blob/ba44ce1/src/tunnel.c#L233-L245

We should avoid offering these options to save an unnecessary round-trip in the
LCP stage of PPP configuration.

Don't blame me. I didn't design this.

For that matter, we don't need to include the asyncmap option with *any*
encapsulation that doesn't use HDLC.

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