Gets a file descriptor out of list.
index_ specifies the index of the file descriptor to get. It is a programmer error for index_ to be out of range; see g_unix_fd_list_get_length().
The file descriptor is duplicated using dup() and set as close-on-exec before being returned. You must call close() on it when you are done.
A possible cause of failure is exceeding the per-process or system-wide file descriptor limit.