linux(4): Handle SO_TIMESTAMPNS socket option

The SO_TIMESTAMPNS enables or disables the receiving of the SCM_TIMESTAMPNS
control message. The cmsg_data field is a struct timespec.
To distinguish between SO_TIMESTAMP and SO_TIMESTAMPNS in the recvmsg()
map the last one to the SO_BINTIME and convert bintime to the timespec.
In the rest, implementation is identical to the SO_TIMESTAMP.

MFC after: 2 weeks