Avoid copying when altering CHAR column in InnoDB table

We do not allow instant character set changes of CHAR columns in InnoDB. Initially, we allowed it for ROW_FORMAT=REDUNDANT as part of MDEV-15563, but that had to be reverted in MDEV-18627.

In the DYNAMIC, COMPACT and COMPRESSED formats, CHAR columns could be instantly extended in those special cases when they are internally stored as variable-length:

• when the column length (chars*mbmaxlen) exceeds 255 bytes
• when using a variable-length character set (mbminlen!=mbmaxlen), such as UTF-8
• when the column type is CHAR(0)

For the REDUNDANT format, it is best to disallow such instantaneous changes for CHAR columns, and let them remain fixed-size, always explicitly storing the same length for every column. The mbminlen!=mbmaxlen optimization was introduced for COMPACT,DYNAMIC,COMPRESSED only.