Ability to hide pk password from command line

[scruel]

Currently we can see pliantext of pk password via command line, and --cache-password won't work for caching --pk_password.

Just make:

ovpncli client.ovpn --pk-password $(/etc/openvpn/client.pass)

To:

ovpncli client.ovpn --pk-password-file /etc/openvpn/client.pass

Or:

export OVPN_PK_PASSWORK_FILE=/etc/openvpn/client.pass
ovpncli client.ovpn

Will be fine enough.