Downcase function parameters.

Downcase function parameters.
Doc fixes.

Rewrite to support secondary selection.
(mouse-sel-maintainer-address): New constant.
(mouse-sel-submit-bug-report): New function.
Rename mouse-sel-selection-type to mouse-sel-primary-thing.
(mouse-sel-secondary-thing): New variable.
(mouse-sel-selection-alist): New constant.
(mouse-sel-set-selection-function): Semantics changed.  Value
should now be a function taking two arguments.
(mouse-sel-get-selection-function): Semantics changed.  Value
should now be a function taking one argument.
(mouse-sel-selection-owner-p-function): New variable.
Removed variable mouse-sel-check-selection-function.
Rename mouse-sel-determine-selection-type to
mouse-sel-determine-selection-thing.
(mouse-sel-set-selection): New function.
(mouse-sel-get-selection): New function.
(mouse-sel-selection-owner-p): New function.
(mouse-sel-selection-overlay): New function.
(mouse-sel-selection-thing): New function.
(mouse-sel-region-to-primary): New function.
(mouse-sel-primary-to-region): New function.
(mouse-sel-eval-at-event-end): New macro.
(mouse-sel-determine-selection-thing): Quad-click selects paragraphs.
Removed variable mouse-sel-retain-highlight; use inverse of
transient-mark-mode instead.
(mouse-select-internal): New function.
(mouse-select): Re-written using mouse-select-internal and
mouse-sel-primary-to-region.
(mouse-select-secondary): New function.
(mouse-extend-internal): New function.
(mouse-extend): Re-written using mouse-extend-internal,
mouse-sel-region-to-primary and mouse-sel-primary-to-region.
(mouse-extend-secondary): New function.
(mouse-insert-selection-internal): New function.
(mouse-insert-selection): Re-written using
mouse-insert-selection-internal.
(mouse-insert-secondary): New function.
(mouse-sel-validate-selection): Check all selections in
mouse-sel-selection-alist.