use setup_tun callback to defer printing connection status AND backgrounding until...

use setup_tun callback to defer printing connection status AND backgrounding until tun_is_up

This will make scripted use of OpenConnect a lot less sensitive to timing of the tunnel
coming up, if a script is trying to use the tunnel as soon as the main process exits.
(See https://gitlab.com/openconnect/openconnect/-/issues/117 for examples.)

Here's a log of OpenConnect connecting to a GlobalProtect server where ESP
fails to start succesfully due to a firewall blocking UDP. With this
change, it doesn't print the connection status or go to background until after the
attempt to connect ESP has failed, and the tunnel has been started.

    $ echo PASSWORD | sudo ./openconnect -u USERNAME vpn.company.com/gateway --prot=gp --passwd-on-stdin -b \
                           -s 'echo +++ vpnc-script called with reason $reason'
    POST https://vpn.company.com/ssl-vpn/prelogin.esp?tmp=tmp&clientVer=4100&clientos=Linux
    Connected to 1.2.3.4:443
    SSL negotiation with vpn.company.com
    Connected to HTTPS on vpn.company.com with ciphersuite (TLS1.2)-(RSA)-(AES-256-GCM)
    Enter login credentials
    POST https://vpn.company.com/ssl-vpn/login.esp
    POST https://vpn.company.com/ssl-vpn/getconfig.esp
    Session will expire after 1440 minutes.
    Tunnel timeout (rekey interval) is 180 minutes.
    Idle timeout is 180 minutes.
    No MTU received. Calculated 1214 for ESP tunnel
    POST https://vpn.company.com/ssl-vpn/hipreportcheck.esp
    Delaying tunnel for 1000 ms with reason: awaiting GPST ESP connection
    Delaying tunnel for 1000 ms with reason: awaiting GPST ESP connection
    Delaying tunnel for 1000 ms with reason: awaiting GPST ESP connection
    Delaying tunnel for 1000 ms with reason: awaiting GPST ESP connection
    Delaying tunnel for 1000 ms with reason: awaiting GPST ESP connection
    Delaying tunnel for 1000 ms with reason: awaiting GPST ESP connection
    Failed to connect ESP tunnel; using HTTPS instead.
    Connected as 10.0.1.2, using SSL, with ESP unsuccessful
    Continuing in background; pid 1234
    +++ vpnc-script called with reason pre-init
    +++ vpnc-script called with reason connect
    $

Here's an example of attempted DTLS connecting on an AnyConnect VPN, where DTLS
never succeeds. This right away gives us some good feedback that we could probably
reduce the duration of the loop:

    Connected to 1.2.3.4:443
    SSL negotiation with vpn.company.com
    Server certificate verify failed: signer not found
    Connected to HTTPS on vpn.company.com with ciphersuite (TLS1.2)-(ECDHE-RSA-SECP384R1)-(AES-256-GCM)
    Got CONNECT response: HTTP/1.1 200 OK
    CSTP connected. DPD 30, Keepalive 20
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    DTLS handshake failed: Resource temporarily unavailable, try again.
    Delaying tunnel for 1000 ms with reason: DTLS MTU detection
    Connected as 10.0.1.2, using SSL, with DTLS in progress

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