Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

mail-parse.el (mail-header-encode-parameter): Define as rfc2045-encode-string.
nnheader.el (nnheader-insert-nov): Protect against junk appearing in the extra mail headers.
gnus-html.el: Prefetch and html washing additions.
gnus-html.el (gnus-html-prefetch-images): Fix up the url-retrieve calling conventions so that prefetch doesn't bug out.
Pass proper format strings to gnus-message.
nnimap.el: Allow anonymous login.
nnimap.el (nnimap-transform-headers): The chars header is called Chars not Bytes.
nnimap.el (nnimap-wait-for-response): Don't infloop if the IMAP connection drops.
gnus-start.el (gnus-get-unread-articles): Call `gnus-open-server' on each method before trying to scan them etc.
gnus-sum.el (gnus-summary-update-mark): Replace subst-char-in-region by subst-char-in-region.
gnus.el (gnus-similar-server-opened): Refactor a bit and add comments.
gnus.el: Fix a speed regression based in methods that were similar weren't the same.
gnus.el (gnus): When using the development version of Gnus, load the gnus-load file.
nnimap.el (nnimap-open-connection):  When looking for credentials, also use the nnimap-server-port.
nnimap.el (nnimap-request-article): Return the group/article number, so that Gnus `^' works as expected.
nnimap.el (nnimap-find-wanted-parts-1): Return the MIME parts as IMAP wants them.
gnus-start.el (gnus-ignored-newsgroups): Remove [] from the list of bogus characters.
gnus-html.el (gnus-html-image-fetched): Protect against the data not arriving.
nnimap.el (nnimap-wait-for-connection): Avoid a race condition while waiting for the connection string.
gnus.texi (Required Back End Functions): Document INFO.