(gnus-signature-toggle): Don't hide the following parts.

(gnus-signature-toggle): Don't hide the following parts.
(gnus-article-highlight-signature): Work for forwarded messages.
(gnus-article-show-all): New function based on `article-show-all'.
(gnus-article-show-all-headers): Based on `article-show-all-headers'.
(article-show-all-headers): New function to show all *HEADERS*.
(article-show-all): Show *ALL* literally.
(article-hide-signature): Work for forwarded messages.
(gnus-treatment-function-alist): Put `gnus-treat-hide-signature' off after
`gnus-treat-highlight-signature'.