fixed globbing of non-existing directories

 * for non-existing directories, NULL was passed to g_dir_read_name().
   This resulted in Glib errors being printed to stdout/stderr.
 * this was broken in commit 427c9d
fixed remaining reference to register "0" in the documentation
updated TODO
add workaround for missing g_spawn_check_exit_status() in libglib v2.33 and earlier

 * Debian 7 is still at libglib v2.33 and since it should be
   supported, I reimplemented the missing function (UNIX-only).
 * This workaround can be removed once libglib v2.34 becomes common place.

Closes #2