Make innodb_max_dirty_pages_pct my.cnf variable a double

Made innodb_max_dirty_pages_pct my.cnf variable a double

Summary:
Feature: Buffer Pool Control Enhancements

This change is to fix: http://bugs.mysql.com/62534

This makes innodb_max_dirty_pages_pct a double with min,default,max values
0.001, 75, 99.999.

This also makes innodb_max_dirty_pages_pct_lwm and adaptive_flushing_lwm
doubles, as these sysvars are inter-dependent.

Added more to the BUFFER POOL AND MEMORY section of SHOW INNODB STATUS:
Percent pages dirty: X.X
This is all n_dirty_pages / used_pages
Percent all pages dirty: X.X
This is all n_dirty_pages / all-pages
Max dirty pages percent: X.X
This is innodb_max_dirty_pages_pct

Also changed all of buf from 2 to 3 digits of precision (%.2f -> %.3f).