Add -M option to nc(1), which makes it print the TCP connection
statistics obtained with stats(3) in JSON format to standard error.

Reviewed by: allanjude, thj, cem (earlier version)
Tested by: thj
MFC after: 2 weeks
Relnotes: yes
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D21324