Use counter(9) for PLPMTUD counters

[kbowling]

Summary

The int increments are not consistent between loads/stores with multiple threads and may lose events during concurrent updates. tcpstats solves that problem with no real downsides.

[jtl]

Looks good (with one minor nit inline).

usr.bin/netstat/inet.c

760

Minor (yes, very minor) nit: On these three lines, the plural should probably be conditioned on the number. %s does this for you automatically.

So, for example "...detection activation%s}\n".

[kbowling]

Use the libxo plurals and fix the pmtud-failed label

[kbowling]

Noob question, should freebsdversion bump since this alters the ABI?