openvpn: Add systemd startup notification

This causes OpenVPN services to reach the "active" state when the VPN
connection is up (i.e., after OpenVPN prints "Initialization Sequence
Completed"). This allows units to be ordered correctly after openvpn-*
units, and makes systemctl present a password prompt:

  $ start openvpn-foo
  Enter Private Key Password: *************

(I first tried to implement this by calling "systemd-notify --ready"
from the "up" script, but systemd-notify is not reliable.)