Renamed Atari drivers.

As a preparation for the introduction of the 'atarixl' target it is
necessary that all drivers have a base name...
- no longer than 7 characters
- with a common prefix
Introduced target 'atarixl'.

The target 'atarixl' is to be used for Atari XL (and better) machines.
It will disable the OS ROM and enable the Shadow RAM available on
those machine.

Note: This commit is only the inital step towards for this goal that just
replicates the target 'atari' as a starting point!
Brought back mouse driver source code.

The mouse driver source code in question was removed two years
ago with commit 6cbbe66c87fc19c322a0dd72a5c8ef094ea90c90.
It seems however desirable to have it around visible in the source
code tree for reference purposes.
Fixed discrepancy between definition and declaration.
Use cbm_load() to load overlays on CBMs.