Recognize more format variation. Automatically reshow decrypted text.

Recognize more format variation.  Automatically reshow decrypted text.

* mail/rmail.el	(rmail-epa-decrypt): Disregard <pre> before armor.
Ignore more kinds of whitespace in mime headers.
Modify the decrypted mime part's mime type so it will be displayed
by default when visiting this message again.

* lisp/mail/rmail.el (rmail-show-message-1): When displaying a mime message,
indicate start and finish in the echo area.