Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Properly disable LUA_USE_DLOPEN for bootstrap flua

flua does have some specific bits that will include luaconf.h, but the
definition of LUA_USE_DLOPEN for those won't matter. This belongs in liblua
instead.

To expand on my previous commit, which was a little sparse with details,
it's not really safe to allow LUA_USE_DLOPEN with host lib paths being used.
The host system could have an entirely different lua version and this could
cause us to crash and burn.

If we want to revive this later, we need to make sure to define c module
paths inside OBJDIR that are compiled against whatever version we've
bootstrapped.

Pointy hat: kevans

liblua: ensure that "require" will fail in bootstrap flua

We do not want to support bootstrapping lua modules, so ensure that
require will fail by providing a nonexistent path.

Reviewed by: kevans
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24610

Add an internal liblua and use it in flua.

The new liblua will be used in a forthcoming import of kyua.

Reviewed by: kevans
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24090