openconnect: allow specify --protocol from config

openconnect: allow specify --protocol from config

openconnect v8.10 supports 4 VPN protocols

  --protocol=anyconnect  Compatible with Cisco AnyConnect SSL VPN, as well as ocserv (default)
  --protocol=nc          Compatible with Juniper Network Connect
  --protocol=gp          Compatible with Palo Alto Networks (PAN) GlobalProtect SSL VPN
  --protocol=pulse       Compatible with Pulse Connect Secure SSL VPN

This patch allows user to specify protocol use the new "vpn_protocol"
option and deprecate the old option "juniper" which seems to be missing in
the current openconnect client.

Signed-off-by: Mengyang Li <redacted>