luajit flowvar support

This patch adds flowvar support to luajit. It does so by exposing two special
C functions to the luajit scripts: ScFlowvarGet and ScFlowvarSet.
flowvar/flowint: make local function static
flowvar/flowint: split set functions into normal and NoLock version, where the latter won't lock the flow.
luajit: add flowint support

Expose ScFlowintGet and ScFlowintSet functions to luajit. These set
flowints in real time, regardless of rule and/or script match.

Example:

function init (args)
    local needs = {}
    needs["http.request_headers"] = tostring(true)
    needs["flowint"] = {"cnt"}
    return needs
end

function match(args)
    a = ScFlowintGet(0);
    if a then
        ScFlowintSet(0, a + 1)
    else
        ScFlowintSet(0, 1)
    end

    a = ScFlowintGet(0);
    if a == 23 then
        return 1
    end

    return 0
end

return 0

Script's init call first registers "cnt" at id 0, then 0 is used to use
this var.
flowvar/luajit: make 'sets' real time. Needed for cross HTTP-header matching.
luajit/flowint: add ScFlowintIncr & ScFlowintDecr

Add flowint lua functions for incrementing and decrementing flowints.

First use creates the var and inits to 0. So a call:

    a = ScFlowintIncr(0)

Results in a == 1.

If the var reached UINT_MAX (2^32), it's not further incremented. If the
var reaches 0 it's not decremented further.

Calling ScFlowintDecr on a uninitialized var will init it to 0.

Example script:

    function init (args)
        local needs = {}
        needs["http.request_headers"] = tostring(true)
        needs["flowint"] = {"cnt_incr"}
        return needs
    end

    function match(args)
        a = ScFlowintIncr(0);
        if a == 23 then
            return 1
        end

        return 0
    end
    return 0

This script matches the 23rd time it's invoked on a flow.