Some VPN providers require username and password for client to connect.
This commit adds an option to specify username, password and
cert_password directly in uci config which then gets expanded during
start of openpvn client.

Signed-off-by: Michal Hrusecky <michal.hrusecky@turris.com>

New features:
* Per client tls-crypt keys
* ChaCha20-Poly1305 can be used to encrypt the data channel
* Routes are added/removed via Netlink instead of ifconfig/route
  (unless iproute2 support is enabled).
* VLAN support when using a TAP device

Significant changes:
* Server support can no longer be disabled.
* Crypto support can no longer be disabled, remove nossl variant.
* Blowfish (BF-CBC) is no longer implicitly the default cipher.
  OpenVPN peers prior to 2.4, or peers with data cipher negotiation
  disabled, will not be able to connect to a 2.5 peer unless
  option data_fallback_ciphers is set on the 2.5 peer and it contains a
  cipher supported by the client.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>