Tramp code cleanup

* lisp/net/tramp.el (tramp-use-absolute-autoload-file-names)
(tramp-skeleton-file-name-all-completions)
(tramp-parse-auth-sources, tramp-parse-file, tramp-parse-netrc):
* lisp/net/tramp-cache.el (tramp-parse-connection-properties):
* lisp/net/tramp-gvfs.el (tramp-gvfs-parse-device-names):
* lisp/net/tramp-sh.el (tramp-sh-handle-insert-directory):
Use `seq-uniq'.

* lisp/net/tramp.el (tramp-register-foreign-file-name-handler):
* lisp/net/tramp-fuse.el (tramp-fuse-handle-directory-files):
Use `seq-union'.

* test/lisp/net/tramp-archive-tests.el
(tramp-archive-test17-insert-directory):
* test/lisp/net/tramp-tests.el (tramp-test17-insert-directory):
Use `seq-union'.
(tramp-test42-utf8): Use `seq-uniq' and `tramp-compat-seq-keep'.

test/lisp/net/tramp-archive-tests.el

@@ -559,7 +559,7 @@ This checks also `file-name-as-directory', `file-name-directory',
 
   (let (;; We test for the summary line.  Keyword "total" could be localized.
 	(process-environment
-	 (append '("LANG=C" "LANGUAGE=C" "LC_ALL=C") process-environment)))
+	 (seq-union '("LANG=C" "LANGUAGE=C" "LC_ALL=C") process-environment)))
     (unwind-protect
 	(progn
 	  (with-temp-buffer

test/lisp/net/tramp-tests.el

@@ -3515,7 +3515,7 @@ This tests also `file-directory-p' and `file-accessible-directory-p'."
 	   (tmp-name2 (expand-file-name "foo" tmp-name1))
 	   ;; We test for the summary line.  Keyword "total" could be localized.
 	   (process-environment
-	    (append '("LANG=C" "LANGUAGE=C" "LC_ALL=C") process-environment)))
+	    (seq-union '("LANG=C" "LANGUAGE=C" "LC_ALL=C") process-environment)))
       (unwind-protect
 	  (progn
 	    (make-directory tmp-name1)
@@ -5656,8 +5656,7 @@ If UNSTABLE is non-nil, the test is tagged as `:unstable'."
 (ert-deftest tramp-test30-make-process ()
   "Check `make-process'."
   :tags (append '(:expensive-test :tramp-asynchronous-processes)
-		(and (getenv "EMACS_EMBA_CI")
-                     '(:unstable)))
+		(and (getenv "EMACS_EMBA_CI") '(:unstable)))
   (skip-unless (tramp--test-enabled))
   (skip-unless (tramp--test-supports-processes-p))
 
@@ -7887,8 +7886,8 @@ This requires restrictions of file name syntax."
        "🌈🍒👋")
 
       (when (and (tramp--test-expensive-test-p) (not (tramp--test-windows-nt-p)))
-	(delete-dups
-	 (mapcar
+	(seq-uniq
+	 (tramp-compat-seq-keep
 	  ;; Use all available language specific snippets.
 	  (lambda (x)
 	    (and