Provide the vpnc-script with our PID (as $VPNPID)

Provide the vpnc-script with our PID (as $VPNPID)

This will enable a vpnc-script to more easily identify which VPN connection
is calling it, in the case of multiple concurrent or "stacked" VPN
connections.

Because OpenConnect (and vpnc) invoke the vpnc-script via an intermediate
shell process, the vpnc-script would otherwise need to determine its
GRANDparent PID, which is an error-prone, and not easily portable, process.
See https://gitlab.com/openconnect/vpnc-scripts/-/issues/28 and
https://gitlab.com/openconnect/vpnc-scripts/-/merge_requests/36 for issues
with the current approaches.

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