When compressed myisam files are opened, they are always memory mapped
sometimes causing memory swapping problems.

When we mmap the myisam compressed tables of size greater than the memory 
available, the kswapd0 process utilization is very high consuming 30-40% of 
the cpu. This happens only with linux kernels older than 2.6.9

With newer linux kernels, we don't have this problem of high cpu consumption
and this option may not be required.
 
The option 'myisam_mmap_size' is added to limit the amount of memory used for
memory mapping of myisam files. This option is not dynamic.

The default value on 32 bit system is 4294967295 bytes and on 64 bit system it
is 18446744073709547520 bytes.

Note: Testcase only tests the option variable. The actual bug has be to 
tested manually.