The save-entropy script rotates entropy files like logs,
but the entropy data do not really need to be kept in a particular
order, and replacing the oldest file is sufficient.
The proposed change replaces the rotation with a scan in the
[1..entropy_save_num) space that finds the first empty slot,
or the slot of oldest file, and overwrites the file in that slot.
This also fixes an issue that prevents save-entropy from saving
any entropy when there is one non-regular file in any slot as a