This chapter was a very unnatural union of several good tutorials.
Many of the sections in this chapter contained duplicate information
(about FreeBSD 1.1.5.1, mind you).

This commit adds a more substantial introduction section with
information about terminology, cables and ports, kernel configuration
of serial devices, and using stty for serial configuration. The
remaining sections ("terminals", "dial-in service", "dial-out
service", and "setting up a serial console") can then refer to this
earlier section instead of duplicating how to setup /etc/ttys and HUP
init, etc. in each one.

Also contains a couple more markup fixes for <keycode>s.

PR: 19481