Exchange point and mark when set-mark-command.

[Yevgnen]

When the selection is turned into an active region, point > mark is assumed in current version. For example when a defun is selected, point is end-of-defun. But the built-in functions mark-defun places point to beginning-of-defun. The other mark-* command behaves in a similar way. This commit makes easy-kill to be consistent with the built-in commands.