Add header-line to the buffer for editing kill-ring items.

When editing items in the `kill-ring' place a header line at the top of
the edit buffer containing a brief description of key bindings, this
replaces the current message containing the key binding advice.

This patch also adds an abort key binding which closes the edit buffer
without saving any changes back to the `kill-ring'.

Finally, this patch kills the edit buffer when the user finishes editing
rather than just burying it, there seems little point keeping the buffer
around once the user has finished with it.

ChangeLog:

	When editing entries in the `kill-ring' place a header line in the
	edit buffer with a short key guide.  Kill the edit buffer when
	editing is finished rather than just burying it.  Add an abort key
	mapping to allow the edit buffer to be closed without saving the
	changes.