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description, which was already outdated, and refer the users to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.