changed from g_spawn_command_line_async to g_spawn_async, so we can get
the PID of the child process

Added a bunch of TODOs...

  * create a table in the registry to store the Lua callback handler
    functions
  * add an entry in that callbacks table for each spawned process

  * made the lua callback function argument to luaH_luakit_spawn
    optional
  * ensure that the type of the element received as second argument is a
    function

now the function defined as callback receives 2 arguments:

  * the termination type: one of either: normal exit (NORMAL_EXIT),
    termination by a signal (TERM_SIGNAL), or unknown reason
    (TERM_UNKNOWN)
  * the termination number. When the process terminated normally, it
    contains the return code of the process. When terminated by a
    signal, it contains the signal number.

3 new items added to the `luakit` namespace:

  luakit.PROC_TERM_EXIT
  luakit.PROC_TERM_SIGNAL
  luakit.PROC_TERM_UNKNOWN

those descriptors will be used to pass the std{out,err} to the Lua
callback function as text.

… callback

… the lua callback"

This reverts commit b0fe8e2.

This reverts commit b12644f.

This reverts commit be234e9.

Conflicts:

	luah.c