Commands and mode for managing multiple eww buffers

Fixes: debbugs:19131

* doc/misc/eww.texi (Basics): Document managing multiple eww buffers.

* lisp/net/eww.el (eww-buffers-mode): New major mode.
(eww-list-buffers, eww-buffer-select, eww-buffer-show-next)
(eww-buffer-show-previous, eww-buffer-kill, eww-buffer-show): New
commands/functions.
* etc/NEWS: Mention the new eww `S' command.
Remove unused data from gnutls_certificate_details

* src/gnutls.c (gnutls_certificate_details): The :signature isn't
that useful, so remove it.
(gnutls_certificate_details): Clean up whitespace slightly.
(Fgnutls_peer_status): Return the DH prime bits for the connection
Put back :signature-algorithm into gnutls.c

(gnutls_certificate_details): Put :signature-algorithm back again,
which was removed by mistake.
`network-security-level' documentation simplification

* lisp/net/nsm.el (network-security-level): Remove the detailed
description, which was already outdated, and refer the users to
the manual.
* src/gnutls.c: Add Windows specs for gnutls_dh_get_prime_bits.
(nsm-check-protocol): Check for weak Diffie-Hellman prime bits.

Fixes: debbugs:19153
* lisp/emacs-lisp/avl-tree.el: Use lexical-binding and cl-lib.

(avl-tree--root): Remove redundant defsetf.
Fatal GnuTLS errors are now silent by default

Fixes: debbugs:16253

(emacs_gnutls_handle_error): Fatal errors should be on level 1, so
that they are not messaged by default.
Various minor details accumulated over time

* lisp/info.el (Info-mode-map): Remove left-over binding.
* lisp/net/tramp.el (tramp-handle-make-symbolic-link): Mark unused arg.
* lisp/obsolete/gulp.el (gulp-create-m-p-alist): Remove unused var `mnt-tm'.
* lisp/progmodes/gud.el (gud-gdb-completions): Remove unused var `start'.
* lisp/url/url-http.el (url-http-activate-callback): Make debug more verbose.
* src/process.c: Whitespace and line-break nitpicks.
(Fgnutls_error_fatalp): Doc fix (bug#18210)

Fixes: debbugs:18210