ignore authenticated received chain (arc), and such headers in rmail

[Alfred M. Szmidt]

This ignores some new headers that have become popular by some smtp
providers which are very verbose.  They are not very useful for the
user, and just polute the buffer.

diff -u --label /home/ams/emacs/lisp/mail/rmail.el --label \#\<buffer\ 
rmail.el\> /home/ams/emacs/lisp/mail/rmail.el /tmp/buffer-content-363vYs
--- /home/ams/emacs/lisp/mail/rmail.el
+++ #<buffer rmail.el>
@@ -363,6 +363,7 @@
          "\\|^importance:\\|^envelope-to:\\|^delivery-date\\|^openpgp:"
          "\\|^mbox-line:\\|^cancel-lock:"
          "\\|^DomainKey-Signature:\\|^dkim-signature:"
+         "\\|^ARC-.*:"
+         "\\|^Received-SPF:"
+         "\\|^Authentication-Results:"
          
"\\|^resent-face:\\|^resent-x.*:\\|^resent-organization:\\|^resent-openpgp:"
          "\\|^x-.*:"))
   "Regexp to match header fields that Rmail should normally hide.

Diff finished.  Sun May  7 12:48:41 2017