WL#903: Improved online variable setting

There are two issues with handling variables today which makes users a bit
frustrated.

1) missleading error message:

mysql> set global innodb_buffer_pool_size=1;
ERROR 1193: Unknown system variable 'innodb_buffer_pool_size'

this is variable but it is just not live settable, which is what error message 
shall say

[ Noted added 2010-06-15 ]
The message now is:
ERROR 1238 (HY000): Variable 'innodb_buffer_pool_size' is a read only variable


2) No good way to quess which variables can be set and which can't.
The good way might be to add "SHOW EXTENDED VARIABLES" which would show 
  - variable name
  - variable type (GLOBAL/SESSION)
  - if it can be set online 
  - if it is readonly (can't be set directly but rather some explicit way), such
as have_innodb
  - variable physical type (string/number etc)
  - minumum and maximum values allowed
  - short description (it would be good to make one match help one)