Gets a file descriptor out of this.
index_ specifies the index of the file descriptor to get. It is a programmer error for
index_ to be out of range; see
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.
the index into the list
the file descriptor, or -1 in case of error