Implement emulator netns support for TCP and UDP
Implement netns option for TCP and UDP
Implement netns for SCTP + bugfixes
Make netns option value a string
Rudimentary test
Document socket option 'netns'
Merge branch 'raimo/linux-network-namespace-sockopt/OTP-11157' into maint

* raimo/linux-network-namespace-sockopt/OTP-11157:
  Document socket option 'netns'
  Rudimentary test
  Make netns option value a string
  Implement netns for SCTP + bugfixes
  Implement netns option for TCP and UDP
  Implement emulator netns support for TCP and UDP