Core: IPsec support improvements

[travier-anssi]

• [ ] Create generic units or daemon for arbitrary tunnel and network namespace configuration
  • [ ] Dynamic network setup (DHCP & DNS) over IPsec
• [ ] Add network namespace for physical interfaces to remove all network access by default
  • [ ] Bridge physical network interfaces as workaround for XFRM interface restrictions
• [ ] IPv6 support
• [ ] Test infrastructure improvements:
  • [ ] DNS & DHCP served over IPsec for clients