Improve behavior of `lost-selection-mode' with multiple buffers

Improve behavior of `lost-selection-mode' with multiple buffers

* etc/NEWS: Announce new hook `post-select-region-hook'.

* lisp/select.el (lost-selection-last-region-buffer): New
variable.
(lost-selection-post-select-region-function): New function.
Deactivate the mark if the buffer changed.
(lost-selection-mode): Add new hook.

* src/keyboard.c (command_loop_1): Run that hook when
appropriate.
(syms_of_keyboard): New hook `post-select-region-hook'.