Make `message-beginning-of-line' aware of folded headers

Make `message-beginning-of-line' aware of folded headers

* lisp/gnus/message.pl (message-beginning-of-header): New function which
moves point to the beginning of a mail header.  The function is aware of
folded headers and with non-nil argument looks for the true beginning of
a header while with nil argument moves to the indented text of header's
value.
(message-beginning-of-line): Function is now aware of folded headers and
either moves point to the indention of a header or, in visual-line-mode,
searches for the beginning of the header.