flua: Add minimal man page

Flua is a minimal Lua interpreter integrated into the FreeBSD base
system. It is intended for internal use and is not designed for
general-purpose scripting or use by third-party applications, and was
originally intentionally undocumented.

There have been questions about its purpose, and questions about the
available functionality from internal users, so provide a minimal man
page including these details.

Reviewed by: kevans, Isaac Freund <ifreund@freebsdfoundation.org>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49835

(cherry picked from commit 550289771d5cb73befb6a1645fb9cdf0b9fb65a7)