integrate obfsvpn

https://0xacab.org/leap/obfsvpn

replacement for https://0xacab.org/leap/shapeshifter

depending on obfsvpn#18 (closed)

• adapt build script, integration into bitmask-core, our golang core lib for android
• update to go v1.17

- [ ] remove shapeshifter from bitmask-core

• adapt vpn setup, try to use --socks-proxy option
• connect to old obfs4 gateways with obfsvpn
• connect to kcp enabled gateways with obfsvpn

- [ ] remove mutual exclusion of UDP connections and obfs4