devel/libks: Add new port

Add a new devel/libks port for SignalWire's libks support library.

libks provides common C utility code used by SignalWire and related
projects, including platform abstraction, threading, networking, JSON,
UUID, TLS, logging, buffers, and general helper routines.

Reviewed by: jrm
Differential Revision: https://reviews.freebsd.org/D56907