Improve usability of pending changelists. Fixes #181.

* Add help text (to all commands defined with defp4cmd*) explaining
  that a prefix argument prompts for command options.
* Include the one-line help text and the note about prefix arguments
  even if the full help text is loaded from "p4 help".
* Change forms are now in p4-change-form-mode, which has additional
  keybindings C-c C-s for committing the form via "p4 submit -i" and
  C-c C-p for committing the form via "p4 change -i" (regardless of
  how you got the form in the first place).
* Change forms document the new keybindings in the header text.