Make json_pack/json_unpack() recursive

Note that we pass va_list pointers around instead of just va_lists, which
would seem more intuitive. This is necessary since the behaviour of va_lists
passed as function parameters is finicky. Quoth stdarg(3):

	If ap is passed to a function that uses va_arg(ap,type) then the value
	of ap is undefined after the return of that function.

The pointer-passing strategy is used by Python's Py_BuildValue() for the same
purpose.