openvpn routing, split tunneling, connection resilience

[cachebag]

part of #288.

routing

add to OpenVpnConfig:

pub redirect_gateway: bool,
pub routes: Vec<VpnRoute>,

redirect_gateway maps to ipv4.never-default = false. custom routes map to ipv4.route-data.

resilience options

pub ping: Option<u32>,
pub ping_exit: Option<u32>,
pub ping_restart: Option<u32>,
pub reneg_seconds: Option<u32>,
pub connect_timeout: Option<u32>,

data ciphers (NCP)

pub data_ciphers: Option<String>,        // e.g. "AES-256-GCM:AES-128-GCM"
pub data_ciphers_fallback: Option<String>,
pub ncp_disable: bool,

add builder methods, wire into settings construction.

tests

unit tests for route serialization, resilience option mapping, cipher negotiation keys.