Defuns for files, commenting and killing by defun

Defuns for files, commenting and killing by defun

File Defuns: Move file-related defuns to their own provision, require it in
init.  Add defuns to copy a file and put its modified date in the filename.

Comment/kill by defun: Make comment-defun respect arg for its call to
mark-defun.  Add a defun to kill by defun, also using mark-defun and its arg.
Bind this to C-M-S-K.