delay_tunnel_reason and delay_close

This is becoming one of the more complicated and interesting parts of the PPP branch.

Factored out into a separate feature branch because I think it deserves separate consideration.

Adding these appears to have several advantages:

• Much clearer diagnostics when something goes wrong in the connection process, since openconnect won't background itself until the tunnel is up. See 557ac6cf commit message for examples.
• Predictable timing of when openconnect -b will exit, when run in a script. Tunnel is guaranteed to be up before subsequent commands may run and try to use the tunnel (see #117).
• Predictable timing of when openconnect will call the vpnc-script with reason=connect (see #117).