org: Allow spaces in #+LINK abbreviation definitions

* lisp/org.el (org-set-regexps-and-options): Allow spaces when
defining link abbreviations via #+LINK keyword.
* testing/lisp/test-org-element.el (test-org-element/link-parser): Add
a new test.
* doc/org-manual.org (Link Abbreviations): Add example demonstrating
link abbreviation with spaces.

Reported-by: Rudolf Adamkovič <salutis@me.com>
Link: https://orgmode.org/list/87zgf7zujc.fsf@localhost

doc/org-manual.org

@@ -3652,8 +3652,11 @@ can define them in the file with
 #+begin_example
 ,#+LINK: bugzilla  https://10.1.2.9/bugzilla/show_bug.cgi?id=
 ,#+LINK: duckduckgo https://duckduckgo.com/?q=%s
+,#+LINK: "Nu Html Checker" https://validator.w3.org/nu/?doc=%h
 #+end_example
 
+The abbreviations containing spaces must be quoted.
+
 In-buffer completion (see [[*Completion]]) can be used after =[= to
 complete link abbreviations.  You may also define a Lisp function that
 implements special (e.g., completion) support for inserting such a

lisp/org.el

@@ -4227,7 +4227,11 @@ related expressions."
 	       (delq nil
 		     (mapcar
 		      (lambda (value)
-			(and (string-match "\\`\\(\\S-+\\)[ \t]+\\(.+\\)" value)
+			(and (or
+                              ;; "abbrev with spaces" spec
+                              (string-match "\\`\"\\(.+[^\\]\\)\"[ \t]+\\(.+\\)" value)
+                              ;; abbrev spec
+                              (string-match "\\`\\(\\S-+\\)[ \t]+\\(.+\\)" value))
 			     (cons (match-string-no-properties 1 value)
 				   (match-string-no-properties 2 value))))
 		      (cdr (assoc "LINK" alist))))))

testing/lisp/test-org-element.el

@@ -2007,6 +2007,15 @@ e^{i\\pi}+1=0
 	    (progn (org-mode-restart)
 		   (goto-char (1- (point-max)))
 		   (org-element-property :type (org-element-context))))))
+  ;; Link abbreviation with spaces.
+  (should
+   (equal "https"
+          (org-test-with-temp-text
+              "#+LINK: \"Nu Html Checker\" https://validator.w3.org/nu/?doc=%h
+              [[Nu Html Checker:test]]"
+	    (progn (org-mode-restart)
+		   (goto-char (1- (point-max)))
+		   (org-element-property :type (org-element-context))))))
   ;; Link abbreviation in a secondary string.
   (should
    (equal "https"