Reorganize the deliver function, splitting out logical units into the

Reorganize the deliver function, splitting out logical units into the
separate functions deliver, get_filename, strip_from, and
copy_message.

(get_filename): Move deliver's filename-getting code to this new
function.

(strip_from): Move deliver's busted-from-line stripping code to this
new function.

(copy_message): Move deliver's stdin-to-new-file-copying code to this
new function.

(deliver): Call the above new functions appropriately.