FLUSH LOGS now rotates relay logs,
and a new variable max_relay_log_size.
Plus a very small bit of code cleaning.


libmysqld/lib_sql.cc:
  open_log has no default arguments anymore.
mysql-test/r/rpl_flush_log_loop.result:
  result update now that FLUSH LOGS rotates relay logs.
mysql-test/r/rpl_log.result:
  result update now that FLUSH LOGS rotates relay logs.
mysql-test/r/rpl_rotate_logs.result:
  result update now that max_binlog_size is 4096.
mysql-test/t/rpl_rotate_logs-master.opt:
  now max_binlog_size must be a multiple of 4096 (see change in mysqld.cc)
sql/log.cc:
  Got rid of default arguments of various MYSQL_LOG methods (the default arguments
  made code reading uneasy).
  Set max_size in ::init().
  New function set_max_size() to set max_size of a MYSQL_LOG on-the-fly.
  More DBUG info.
sql/mysql_priv.h:
  no defaults in open_log().
  New variables max_relay_log_size.
sql/mysqld.cc:
  New variable and option max_relay_log_size.
  max_binlog_size and max_relay_log_size are multiples of IO_SIZE.
  No more default arguments for log functions.
sql/set_var.cc:
  New variable max_relay_log_size.
  If it is 0, then max_binlog_size will apply to relay logs.
  When one of these variables is changed, fix_max_%log_size is called
  to update max_size of the binary and/or relay logs.
sql/slave.cc:
  New function rotate_relay_log().
sql/slave.h:
  New function rotate_relay_log().
sql/sql_class.h:
  New member max_size of MYSQL_LOG (for automatic rotation).
  New method set_max_size() for setting on-the-fly.
sql/sql_parse.cc:
  Flush the relay log in FLUSH LOGS.