Add variable `scroll-preserve-screen-position-commands'.

Add variable `scroll-preserve-screen-position-commands'.
http://lists.gnu.org/archive/html/emacs-devel/2010-04/msg00403.html

* window.c (Vscroll_preserve_screen_position_commands): New variable
with the default value as the list of Qscroll_down and Qscroll_up.
(window_scroll_pixel_based, window_scroll_line_based): Search the
last command in the list Vscroll_preserve_screen_position_commands
instead of comparing with Qscroll_up and Qscroll_down.

* mwheel.el (scroll-preserve-screen-position-commands):
Add mwheel-scroll to this list of commands.

* simple.el (scroll-preserve-screen-position-commands):
Add scroll-up-command, scroll-down-command, scroll-up-line,
scroll-down-line to this list of commands.