flua: add fbsd module

This module is bundled into flua, it only provides for now the exec
function. The point of the function is to be able to execute a program
without actually executing a shell.

to use it:
fbsd.exec({"id", "bapt"})

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D41840

Remove $FreeBSD$: one-line .c comment pattern

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

flua: add ucl library

libucl comes with a Lua library binding. Build it into flua.

This lets us parse/generate config files in the various formats supported by
libucl with flua. For example, the following script will detect the format of
an object written to stdin as one of UCL config, JSON, or YAML and write it to
stdout as pretty-printed JSON:

local ucl = require('ucl')
local parser = ucl.parser()
parser:parse_string(io.read('*a'))
local obj = parser:get_object()
print(ucl.to_format(obj, 'json'))

Reviewed by: kevans, pstef
Approved by: mmacy (mentor)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D25009

flua: implement chmod

Lua does not provide a native way to change the permission of a file.

Submitted by: Yang Wang <2333@outlook.jp>
Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24036

Add flua to the base system, install to /usr/libexec

FreeBSDlua ("flua") is a FreeBSD-private lua, flavored with whatever
extensions we need for base system operations. We currently support a subset
of lfs and lposix that are used in the rewrite of makesyscall.sh into lua,
added in r354786.

flua is intentionally written such that one can install standard lua and
some set of lua modules from ports and achieve the same effect.

linit_flua is a copy of linit.c from contrib/lua with lfs and lposix added
in. This is similar to what we do in stand/. linit.c has been renamed to
make it clear that this has flua-specific bits.

luaconf has been slightly obfuscated to make extensions more difficult. Part
of the problem is that flua is already hard enough to use as a bootstrap
tool because it's not in PATH- attempting to do extension loading would
require a special bootstrap version of flua with paths changed to protect
the innocent.

src.lua.mk has been added to make it easy for in-tree stuff to find flua,
whether it's bootstrap-flua or relying on PATH frobbing by Makefile.inc1.

Reviewed by: brooks, emaste (both earlier version), imp
Differential Revision: https://reviews.freebsd.org/D21893