Fix 'max key length is 767 bytes' errors (Closes: #886756)

Allow index keys up to 3072 bytes. Many apps expect VARCAHR(255) to work.
Since we introduced UTF-8, that has not been the case. This can be fixed
via option 'innodb_default_row_format' that was backported to 10.0.x
series recently.

The option 'innodb_file_per_table' is 'on' by defautl since MariaDB 5.5,
so there is no need to define it.

See also
https://jira.mariadb.org/browse/MDEV-14904
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=848603
https://mariadb.com/kb/en/library/xtradbinnodb-server-system-variables/#innodb_file_format