Remove `org-link-protocols' variable

* lisp/org.el (org-link-protocols): Remove variable org-link-protocols.
The data in this variable is now retrieved with org-link-get-parameter.

* lisp/org.el: Enable file links to use the application link property to
  determine the follow action. Also add a space after let*.

lisp/org.el

@@ -10756,10 +10756,10 @@ link in a property drawer line."
 	  ;; When link is located within the description of another
 	  ;; link (e.g., an inline image), always open the parent
 	  ;; link.
-	  (let*((link (let ((up (org-element-property :parent context)))
-			(if (eq (org-element-type up) 'link) up context)))
-		(type (org-element-property :type link))
-		(path (org-link-unescape (org-element-property :path link))))
+	  (let* ((link (let ((up (org-element-property :parent context)))
+			 (if (eq (org-element-type up) 'link) up context)))
+		 (type (org-element-property :type link))
+		 (path (org-link-unescape (org-element-property :path link))))
 	    ;; Switch back to REFERENCE-BUFFER needed when called in
 	    ;; a temporary buffer through `org-open-link-from-string'.
 	    (with-current-buffer (or reference-buffer (current-buffer))
@@ -10767,7 +10767,7 @@ link in a property drawer line."
 	       ((equal type "file")
 		(if (string-match "[*?{]" (file-name-nondirectory path))
 		    (dired path)
-		  ;; Look into `org-link-protocols' in order to find
+		  ;; Look into `org-link-parameters' in order to find
 		  ;; a DEDICATED-FUNCTION to open file.  The function
 		  ;; will be applied on raw link instead of parsed
 		  ;; link due to the limitation in `org-add-link-type'
@@ -10781,7 +10781,9 @@ link in a property drawer line."
 		  (let* ((option (org-element-property :search-option link))
 			 (app (org-element-property :application link))
 			 (dedicated-function
-			  (nth 1 (assoc app org-link-protocols))))
+			  (org-link-get-parameter
+			   (if app (concat type "+" app) type)
+			   :follow)))
 		    (if dedicated-function
 			(funcall dedicated-function
 				 (concat path
@@ -10796,8 +10798,8 @@ link in a property drawer line."
 				    (list (string-to-number option)))
 				   (t (list nil
 					    (org-link-unescape option)))))))))
-	       ((assoc type org-link-protocols)
-		(funcall (nth 1 (assoc type org-link-protocols)) path))
+	       ((functionp (org-link-get-parameter type :follow))
+		(funcall (org-link-get-parameter type :follow) path))
 	       ((equal type "help")
 		(let ((f-or-v (intern path)))
 		  (cond ((fboundp f-or-v) (describe-function f-or-v))
@@ -10846,7 +10848,7 @@ link in a property drawer line."
 		    (user-error "Abort"))))
 	       ((equal type "id")
 		(require 'org-id)
-		(funcall (nth 1 (assoc "id" org-link-protocols)) path))
+		(funcall (org-link-get-parameter type :follow) path))
 	       ((member type '("coderef" "custom-id" "fuzzy" "radio"))
 		(unless (run-hook-with-args-until-success
 			 'org-open-link-functions path)

lisp/ox.el

@@ -4116,7 +4116,7 @@ The function ignores links with an implicit type (e.g.,
   (let ((type (org-element-property :type link)))
     (unless (or (member type '("coderef" "custom-id" "fuzzy" "radio"))
 		(not backend))
-      (let ((protocol (nth 2 (assoc type org-link-protocols))))
+      (let ((protocol (org-link-get-parameter type :export)))
 	(and (functionp protocol)
 	     (funcall protocol
 		      (org-link-unescape (org-element-property :path link))