WL#865: comments for Variables and Status values

Currently we've added comments (descriptions) for charsets and and table types.
It would be nice to have the same for variables and  status statistics counters.

If we would like to preserve old view as default we can have have some flag to
SHOW for this purpose.

One more thing we shall do to make the values usable for external tools is to
supply the type of the value. If this is the additive value (ie connections) or 
the snapshot value (thread_connected). It is typical to have them processed
differently.

Also we shall differ per thread values from global ones (such as key_blocks_used)