* lisp/gnus/message.el: Tweak header font-lock and ecomplete completion
(message-font-lock-make-header-matcher): Delete.
(message-match-to-eoh): New function to replace it.
(message-font-lock-keywords): Use it.
(message-strip-forbidden-properties): Remove redundant binding.
(message-goto-body): Avoid called-interactively-p, only use
push-mark when called interactively.
(message-goto-body-1): Merge into message-goto-body. Redefine as alias.
(message-goto-eoh): Call message-goto-body interactively.
(message--in-tocc-p): New function, extracted from message-display-abbrev.
(message-ecomplete-capf): New function.