Apply the initial log-edit tweaks discussed at emacs-devel

Apply the initial log-edit tweaks discussed at emacs-devel
* .dir-locals.el: (log-edit-move): Add the "Author: " header.

* lisp/vc/log-edit.el (log-edit-mode-map): Add binding for
`log-edit-beginning-of-line'.
(log-edit-setup-add-author): New user option.
(log-edit-beginning-of-line): New command.
(log-edit): Move major mode call above the contents setup so that
the local variable values are already applied.
(log-edit): Only insert "Author: " when
`log-edit-setup-add-author' is non-nil.
(log-edit): When SETUP is non-nil, position point after ": "
instead of point-min.