MINOR: tcp_sample: clarifying samples support per os, for further expansion.

MINOR: tcp_sample: clarifying samples support per os, for further expansion.

While there is some overlap between what each OS provides in terms of
retrievable info, each set is not a real subset of another one and this
results in increasing complexity when trying to add support for new OSes.
Let's just condition each item to the OS that support it. It's not pretty
but at least it will avoid a real mess later.

Note that fc_rtt and fc_rttvar are supported on any OS that has TCP_INFO,
not just linux/freebsd/netbsd, so we continue to expose them unconditionally.