setting of innodb_io_capacity_max

(a) Changed the behaviour so that if you set innodb_io_capacity to a 
value > innodb_io_capacity_max that the value is accepted AND 
that innodb_io_capacity_max = innodb_io_capacity * 2.

(b) If someone wants to reduce innodb_io_capacity_max and 
reduce it below innodb_io_capacity then innodb_io_capacity 
should be reduced to the same level as innodb_io_capacity_max.

In both cases give a warning to user.