flua: fbsd: allow stdout to be captured for exec() processes

This allows us to do things like:

local fp = assert(fbsd.exec({"ls", "-l"}, true))
local fpout = assert(fp:stdout())

while true do
        local line = fpout:read("l")
        if not line then break end
        print("Read: " .. line)
end

fp:close()

The makeman lua rewrite will use it to capture make showconfig output
for processing.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D50539

(cherry picked from commit 3f0e1092097ed63b83a02518395e370c3cac01be)