openconnect: Restore support for vpnc-script hooks

* Restore hooks support from vpnc-script
* Use consistent style for tests and blocks in openconnect.sh
* Cleanup code that writes banner to syslog

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
Merge pull request #677 from sykesm/openconnect-vpnc-hooks

openconnect: Restore support for vpnc-script hooks