* lisp/url/url-http.el (url-http): Make 'url-max-redirections'
buffer-local in request buffers, to make dynamic binding over
'url-retrieve' work as expected.

(Bug#79186)

When building a query string via `url-build-query-string',
%-encode literal % characters appearing in both the keys and
the values.
* lisp/url/url-util.el (url--query-key-value-preserved-chars):
Define a new constant based on `url-query-key-value-allowed-chars'
specifying the characters that should be preserved when %-encoding
query-string keys and values.
(url-build-query-string): Use the new constant (fixes Bug#78984).

* test/lisp/url/url-util-tests.el (url-util-tests): Add a test.

(url-queue): New slots to hold request related variables.
(url-queue-retrieve): Capture the request related variables.
(url-queue-start-retrieve): Let bind the request related
variables.  (Bug#78928)

* etc/NEWS: Announce changes.

Many variables were copied from the dynamic environment to the request
buffer, which allowed them to influence the handling of requests and
responses.  But some were not, notably some of the mime-related
variables, and the user-agent and privacy variables.  This made them
unreliable when dynamically bound around a call to `url-retrieve'; they
would have the desired effect when reusing an existing connection, but
not when url-http opened a new connection.  In the case of reused
connections, the request construction happens in the dynamic scope of
`url-http', but in the case where a fresh connection is needed, request
construction happens outside that dynamic scope.

This commit adds the remaining variables used in request construction to
the set of buffer local variables mirroring the dynamic values from
url-http, and adds a comment describing the mechanism used and how avoid
the pitfall of inconsistent handling of dynamic bindings.

* lisp/url/url-http.el (url-http-extensions-header): New internal-ish
variable.
(url-http-create-request): Use the new variable instead of the global
one.
(url-http-idle-sentinel): Debug-log when the connection closes.
(url-http): Set up more buffer-local variants of dynamic variables in
the buffer used for the request, and add comment describing why this
copying is needed.

(Bug#61916)

* lisp/url/url-http.el (url-http-create-request): Prepend "/" if
it's missing from the filename.  (Bug#78640)

* lisp/url/url-cookie.el (url-cookie-save-interval): Fix doc
string and :type.  (Bug#78303)

* lisp/url/url-parse.el (url-recreate-url): Handle empty filenames
without errors.

* test/lisp/url/url-parse-tests.el
(url-generic-parse-url/ms-windows-file-uri-hanlding): Add one
more test for the scenario above.

* lisp/url/url-parse.el (url-generic-parse-url): Remove prefix /
when a file URI's filename starts with a single letter followed
by a colon and a slash or backslash.
(url-recreate-url): Mirror the change applied when parsing, so
the URL is recreated properly on MS-Windows.
* test/lisp/url/url-parse-tests.el
(url-generic-parse-url/ms-windows-file-uri-hanlding): New test.
(Bug#76982)

Side-companion to commits bb0f84bc0bf7 and 50947fd51202.
While in there, couldn't resist adding a few minor cleanups
I had lying around.

* lisp/savehist.el (savehist-save): Add `lexical-binding:t` cookie.

* lisp/url/url-cookie.el: Remove redundant `:group` arguments.
(url-cookie-write-file): Add `lexical-binding:t` cookie.

* lisp/abbrev.el: Prefer #' to quote function names.
(write-abbrev-file): Add a `lexical-binding:t` cookie.
(unexpand-abbrev): Use `replace-region-contents` and 3-args `<=`.
(define-abbrev-table): Use `defvar-1`.

* lisp/filesets.el (filesets-menu-cache-file-save-maybe):
Add `lexical-binding:t` cookie.
(filesets-ingroup-patterns): Don't quote lambdas.
(filesets-spawn-external-viewer): Remove redundant "" arg to `mapconcat`.

* lisp/recentf.el: Prefer #' to quote function names.
(recentf-save-file-header): Add `lexical-binding:t` cookie and change
mode to match the actual content.