Hello,

This patch adds a local version of `delete-selection-mode`. I use a 
modal editing package for which I would like to have 
`delete-selection-mode` enabled depending on the modal state that is 
active in the buffer. This doesn't currently work because 
`delete-selection-mode` applies to all buffers.

For compatibility, I made the local mode set the value of the variable 
`delete-selection-mode` buffer locally. I see that some features, such 
as the built-in `table.el`, check that variable to adjust their 
behavior. Do you think that is a good way to maintain compatibility?

Thank you.