backup_maintenance using delete_amount is wrong in v16 PHP 7

This bug is serious because it causes the most recent backup to be immediately deleted, meaning the only backups retained are older and older.

backup_maintenance() uses ksort(SORT_NUMERIC) to order the files before removing 'stale' backups in excess of the delete_amount value.

SORT_NUMERIC is a poor choice when using file names.  It gives somewhat unpredictable results, and PHP 7 effectively reserves the sort order compared with PHP 5.

The current workaround is to turn of maintenance by amount/count.  However, if the user isn't aware of this bug, they will think they are doing successful backups, only to find when they need a backup that they only have backups prior to the upgrade to v16.