flua: initial support for "require" in the base system

Use /usr not /usr/local for base system components.

Use /usr/lib/flua and /usr/share/flua (not lua) for consistency and to
avoid the possibility that other software accidentally finds our base
system modules.

Also drop the version from the path, as flua represents an unspecified
lua version that corresponds to the FreeBSD version it comes with.

LUA_USE_DLOPEN is not yet enabled because some additional changes are
needed wrt symbol visibility.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24605