Please backport `innodb_default_row_format` to MariaDB 10.1

Hi there,

MariaDB 10.1 doesn't support `innodb_default_row_format` yet. Since InnoDB in Debian uses `utf8mb4` as default character set since version `10.0.20-2`, this leads to major pain, e.g. due to keys longer than 767 bytes.

A lot of applications silently expect the supported maximum size of VARCHAR to be 255 characters which means 1016 bytes with `utf8mb4`.

In order to support larger keys, `innodb_large_prefix` needs to be enabled and ROW_FORMAT set to `dynamic`. The former can be configured in the MariaDB server configuration in a persistent way, the latter not due to missing configuration option `innodb_default_row_format`.

Backporting `innodb_default_row_format` will at least give the option to server admins to configure MariaDB 10.1 in a way that applications don't break if they want to create keys > 767 bytes.

PS: I already discussed this issue with Marko Mäkelä (dr-m) on IRC.