Improvements to sieve script usability

Improvements to sieve script usability

* lisp/net/sieve-mode.el (sieve-mode-menu): Add
  `sieve-upload-and-kill' to the mode menu.

* lisp/net/sieve.el (sieve-manage-mode-menu): Add `sieve-manage-quit'
  to the mode menu.
  (sieve-help): Mention quitting in mode help.
  (sieve-manage-mode): Directly derive from special-mode.
  (sieve-edit-script): Start off with unmodified buffer.
  (sieve-upload): Set buffer unmodified after successful upload.

Changes are aimed at 1) increasing discoverability of commands and 2)
using buffer modification to help users understand what's happening.