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.

include/my_global.h:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  define SIZE_T_MAX
include/myisam.h:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  declare 'myisam_mmap_size' and 'myisam_mmap_used' variables and the mutex
  THR_LOCK_myisam_mmap
myisam/mi_packrec.c:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  add 'myisam_mmap_size' option which limits the memory available to mmap of 
  myisam files
myisam/mi_static.c:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  declare 'myisam_mmap_size' and 'myisam_mmap_used' variables and the mutex
  THR_LOCK_myisam_mmap
myisam/myisamdef.h:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  move MEMMAP_EXTRA_MARGIN to myisam.h so that it can be used in mysqld.cc
mysql-test/r/variables.result:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  Testcase for BUG#37408 to test the myisam_mmap_size option
mysql-test/t/variables.test:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  Testcase for BUG#37408 to test the myisam_mmap_size option
mysys/my_thr_init.c:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  intialize the lock THR_LOCK_myisam_mmap
sql/mysqld.cc:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  add the 'myisam_mmap_size' option
sql/set_var.cc:
  Fix for Bug #37408 - Compressed MyISAM files should not require/use mmap()
  
  add the 'myisam_mmap_size' to the SHOW VARIABLES list