Make sonewconn() overflow messages have per-socket rate-limits and values.

sonewconn() emits debug-level messages when a listen socket's queue
overflows. Currently, sonewconn() tracks overflows on a global basis. It
will only log one message every 60 seconds, regardless of how many sockets
experience overflows. And, when it next logs at the end of the 60 seconds,
it records a single message referencing a single PCB with the total number
of overflows across all sockets.

This commit changes to per-socket overflow tracking. The code will now
log one message every 60 seconds per socket. And, the code will provide
per-socket queue length and overflow counts. It also provides a way to
change the period between log messages using a sysctl.

Reviewed by: jhb (previous version), bcr (manpages)
MFC after: 2 weeks
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D24316