Added -S argument for better stack traces.

Uses StackTracePlus: https://github.com/ignacio/StackTracePlus

New look with -S:

    $ sudo ./snabb -S -e 'assert(nil)'
    [string "assert(nil)"]:1: assertion failed!
    line not found

    Stack Traceback
    ===============
    (1) Lua function 'handler' at file 'core/main.lua:137' (best guess)
	    Local variables:
	     reason = string: "[string \"assert(nil)\"]:1: assertion failed!"
	     (*temporary) = C function: print
    (2) global C function 'assert'
    (3) main chunk of [string "assert(nil)"] at line 1
    (4) Lua function 'main' at file 'core/main.lua:66' (best guess)
	    Local variables:
	     args = table: 0x53587ff0  {1:-S, 2:-e, 3:assert(nil)}
	     i = number: 2
	     thunk = Lua function '?' (defined at line 0 of chunk "assert(nil)"])
	     error = nil
    (5) global C function 'xpcall'
    (6) main chunk of file 'core/main.lua' at line 142
    (7) global C function 'require'
    (8) main chunk of [string "require "core.main""] at line 1
	    nil

Old look:

    $ sudo ./snabb -e 'assert(nil)'
    [string "assert(nil)"]:1: assertion failed!
    stack traceback:
	    core/main.lua:137: in function <core/main.lua:135>
	    [C]: in function 'assert'
	    [string "assert(nil)"]:1: in function 'thunk'
	    core/main.lua:66: in function <core/main.lua:44>
	    [C]: in function 'xpcall'
	    core/main.lua:142: in main chunk
	    [C]: in function 'require'
	    [string "require "core.main""]:1: in main chunk

src/core/main.lua

@@ -4,6 +4,7 @@ module(...,package.seeall)
 -- (Can be overridden with -P argument: see below.)
 package.path = ''
 
+local STP = require("lib.lua.StackTracePlus")
 local ffi = require("ffi")
 local zone = require("jit.zone")
 local C   = ffi.C
@@ -28,6 +29,7 @@ Available options are:
 -l name      Require library 'name'.
 -t name      Test module 'name' with selftest().
 -d           Debug unhandled errors with the Lua interactive debugger.
+-S           Print enhanced stack traces with more debug information.
 -jdump file  Trace JIT decisions to 'file'. (Requires LuaJIT jit.* library.)
 -jp          Profile with the LuaJIT statistical profiler.
 -jp=args[,.output]
@@ -66,6 +68,9 @@ function main ()
       elseif args[i] == '-d' then
 	 debug_on_error = true
 	 i = i + 1
+      elseif args[i] == '-S' then
+         debug.traceback = STP.stacktrace
+        i = i + 1
       elseif (args[i]):match("-jp") then
 	 local pargs, poutput = (args[i]):gmatch("-jp=(%w*),?(.*)")()
 	 if poutput == '' then poutput = nil end