Duplicate code cleanup: add function normalize_db_name()

There is a duplicate code in sql_db.cc, in functions mysql_create_db_internal() and mysql_rm_db_internal().

char db_tmp[SAFE_NAME_LEN], *dbnorm;

if (lower_case_table_names)

{

  strmake_buf(db_tmp, db);

  my_casedn_str(system_charset_info, db_tmp);

  dbnorm= db_tmp;

}

else

  dbnorm= db;

Will introduce a new function normalize_db_name() and reuse it.

This change was originally a part of MDEV-11952, but it was decided to commit it as a standalone change.