MFH: r309295

bhyve: stability and performance improvement for dbgport

The TCP server implementation in dbgport does not track clients, so it
may try to write to a disconected socket resulting in SIGPIPE.
Avoid that by setting SO_NOSIGPIPE socket option.

Because dbgport emulates an I/O port to guest, the communication is done
byte by byte. Reduce latency of the TCP/IP transfers by using
TCP_NODELAY option. In my tests that change improves performance of
kgdb commands with lots of output (e.g. info threads) by two orders of
magnitude.

A general note. Since we have a uart emulation in bhyve, that can be
used for the console and gdb access to guests. So, bvmconsole and bvmdebug
could be de-orbited now. But there are many existing deployments that
still dependend on those.

Discussed with: julian, jhb
Sponsored by: Panzura