Protocols should try explicitly request the same IP addresses on reconnect, since...

Protocols should try explicitly request the same IP addresses on reconnect, since they will abort if new addresses are sent by the server.

* GlobalProtect:
  - Supported and used by official clients (POST /ssl-vpn/getconfig.esp with preferred-ip form field).
  - GlobalProtect servers often give different IP addresses on reconnect if this mechanism is *not* used,
    so this mechanism is necessary.
  - Same mechanism appears to exist for IPv6 (preferred-ipv6) and was added to OpenConnect in
    d6db0ec03394234d41fbec7ffc794ceeb486a8f0, even though IPv6 support is not yet complete.
* AnyConnect:
  - Not (yet) supported by ocserv
  - It appears that *some* AnyConnect server will try to provide the IP address provided in the X-CSTP-Address
    *request* header along with the CONNECT request, but other servers appear not to
  - This patch reproduces the behavior of GPST: attempt to request same IPv4 and IPv6 addresses on reconnect,
    via CONNECT headers.
* Juniper:
  - There does not appear to be any way to provide this using the Juniper NC protocol.
  - No known reports of Juniper servers giving out different IP address on reconnect.

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