lib9p: Make it a private library

lib9p was imported to add a 9p server to bhyve (and I believe this was
the original motivation for writing it in the first place). Its
external interfaces are kind of strange (from first-hand experience
using it to implement an inetd-based 9p server) and undocumented.
Moreover, upstream has been inactive for over five years.

I suspect there are no third-party consumers. Let's make it a private
library for now, so as to make it easier to rework external interfaces.
If we get more code written against it, symbol versioning, and some
documentation, we can revisit this decision.

PR: 297499
Reviewed by: jhb, emaste
Differential Revision: https://reviews.freebsd.org/D58828