Binlog lacks encoding info about DROPped temporary table.

Idea of the fix is to switch temporary to system_charset_info when a temporary table
is DROPped for binlog. Since that is the server, that automatically, but not the client, who generates the query
the binlog should be updated on the server's encoding for the coming DROP.
The `write_binlog_with_system_charset()' is introduced to replace similar problematic places in the code.


mysql-test/r/drop_temp_table.result:
  results changed
mysql-test/r/mix_innodb_myisam_binlog.result:
  results changed
mysql-test/r/mysqlbinlog.result:
  results changed
mysql-test/r/rpl_temporary.result:
  results changed
mysql-test/t/mysqlbinlog.test:
  Check roll-forward recovery from binlog where there are DROP temporary tables created
  in koi8r.
mysql-test/t/rpl_temporary.test:
  Check slave digests binlog with DROP temporary tables created in koi8r.
sql/mysql_priv.h:
  `write_binlog_with_system_charset()' is added to be called when a binlog event
  is created "implicitly" like DROP temporary table is case of closing connection.
sql/sql_base.cc:
  Idea of the fix is to switch temporary to system_charset_info when a temporary table
  is DROPped for binlog. Since that is the server, not the client, who generates the query
  the binlog should be updated on server's encoding for the coming DROP.