MacOS can feed fc_rtt, fc_rttvar, fc_sacked, fc_lost and fc_retrans
so let's expose them on this platform.
Note that at the tcp(7) level, the API is slightly different, as
struct tcp_info is called tcp_connection_info and TCP_INFO is
called TCP_CONNECTION_INFO, so for convenience these ones were
defined to point to their equivalent. However there is a small
difference now in that tcpi_rtt is called tcpi_rttcur on this
platform, which forces us to make a special case for it before
other platforms.
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