mariadb-server-10.0 depends on mysql-common.
mariadb-server-10.0 provides /etc/mysql/conf.d/mysqld_safe_syslog.cnf, which has "syslog" option.
mysql-common in debian repo provides /etc/mysql/my.cnf, which has "log_error=" option.
mysql-common in mariadb repo provides /etc/mysql/my.cnf, which has no "log_error=" option.

As a result default debian installation has both "log_error=" and "syslog" options, causing a warning and ignores intended "syslog".
Default installation from mariadb repo works well though.

There was an idea to add something like "disable_log_error" to mysqld_safe_syslog.cnf, but then how do we distinguish between default "log_error" and one specified by user?