WL#2781: Allow to operate without a default key cache

We want to be able to run MyISAM without a default key cache.

SET GLOBAL key_buffer_size= 0; shall disable the default key cache (the key
cache structures cannot be deleted though). The key blocks owned by the default
key cache shall be freed for reuse by other key caches (or even handed back to
the system?).

Here is an excerpt from an IRC discussion of 2005-08-12:

 ingo: about BUG#10473
 One should be able to set the key cache size to zero
...
 MySQL works perfectly way without a key cache and in some case one may
need to do that (for testing)
...
 but I agree we should not show 8 when we don't have a key cache
...
 the thing is that we can't remove the default key cache structure, but
we can disable the cache
...
 ingo: agree that the default code can't handle the normal key cache as
the old blocks can't be reassigned to another key cache
 ingo: ok, lets do it this way: Can you do a worklog entry to allow one
to change default key cache to 0 (this doesn't have to be fixed in 5.0)