Fix logic for returning to and yanking from Rmail buffer.

Fix logic for returning to and yanking from Rmail buffer.

* mail/rmail.el (rmail-start-mail):
Pass (rmail-mail-return...) for the return-action.
Pass (rmail-yank-current-message...) for the yank-action.
(rmail-yank-current-message): New function.
(rmail-mail): Pass the Rmail buffer, not view buffer, for replybuffer.
(rmail-reply): Likewise.
(rmail-forward): Pass the Rmail buffer, not nil, for replybuffer.

* mail/sendmail.el (mail-bury): Choose the first rmail-mode
buffer, not the last.  Reject temp buffers.  Use the rmail-mode
buffer, not newbuf.