4726. [port] Prevent setsockopt() errors related to TCP_FASTOPEN
from being logged on FreeBSD if the kernel does not
support it. Notify the user when the kernel does
support TCP_FASTOPEN, but it is disabled by sysctl.
Add a new configure option, --disable-tcp-fastopen, to
disable use of TCP_FASTOPEN altogether. [RT #44754]
(cherry picked from commit c2179857de)
b351a58647