Rename libmysqld.so to libmariadbd.so

Please rename the dynamic and static libraries libmysql.so/a to libmariadbd.so/a. Provide symlinks for backwards compatibility.

In Debian the rename has already been made. We are not allowed to shipd libmysqld.* in a package called libmariadbd (or libmariadbd-dev). This rename should also be done upstream. It would just "make sense" that the library is called libmariadbd as it ships from MariaDB.

With symlinks we can keep compatibility to the old or MySQL versions, just like we have done in the case of libmariadbclient vs libmysqlclient.