Prefer https and fix broken links in ERC

* lisp/erc/erc-button.el (erc-button-rfc-url)
(erc-button-search-url): Prefer https.
* lisp/erc/erc-capab.el:
* lisp/erc/erc.el (erc-cmd-MODE): Fix broken links.

lisp/erc/erc-button.el

@@ -105,18 +105,19 @@ longer than `erc-fill-column'."
   "Flag indicating whether nicks should be buttonized or not."
   :type 'boolean)
 
-(defcustom erc-button-rfc-url "http://www.faqs.org/rfcs/rfc%s.html"
-  "URL used to browse rfc references.
+(defcustom erc-button-rfc-url "https://tools.ietf.org/html/rfc%s"
+  "URL used to browse RFC references.
 %s is replaced by the number."
-  :type 'string)
+  :type 'string
+  :version "28.1")
 
 (define-obsolete-variable-alias 'erc-button-google-url
   'erc-button-search-url "27.1")
 
-(defcustom erc-button-search-url "http://duckduckgo.com/?q=%s"
+(defcustom erc-button-search-url "https://duckduckgo.com/?q=%s"
   "URL used to search for a term.
 %s is replaced by the search string."
-  :version "27.1"
+  :version "28.1"
   :type 'string)
 
 (defcustom erc-button-alist

lisp/erc/erc-capab.el

@@ -40,8 +40,8 @@
 ;; disable this module, it will continue removing message flags, but the
 ;; unidentified nickname prefix will not be added to messages.
 
-;; Visit <http://freenode.net/faq.shtml#spoofing> and
-;; <http://freenode.net/faq.shtml#registering> to find further
+;; Visit <https://freenode.net/kb/answer/cloaks> and
+;; <https://freenode.net/kb/answer/registration> to find further
 ;; explanations of this capability.
 
 ;; From freenode.net's web site (not there anymore) on how to mark

lisp/erc/erc.el

@@ -3389,7 +3389,7 @@ to send.
 If only one word is given, display the mode of that target.
 
 A list of valid mode strings for Freenode may be found at
-URL `http://freenode.net/using_the_network.shtml'."
+URL `https://freenode.net/kb/all'."
   (cond
    ((string-match "^\\s-\\(.*\\)$" line)
     (let ((s (match-string 1 line)))