Allow innodb_log_files_in_group=1

The InnoDB redo log consists of a list of files that logically form a bigger file, as if the individual files were concatenated together.
If there are multiple files, the first file will always be written on redo log checkpoint, because the two checkpoint pages are at the start of the single logical redo log file.
There is no technical reason why InnoDB requires there to be at least 2 files. Let us reduce the minimum number to 1. In that way, restoring from backups will become easier, since InnoDB could directly deal with the single log file produced by XtraBackup.