Improve drdr suffix-decision code, and the help text for prop setting.

The code is improved, and also the default changes for *.rkt files:
there is no need for any flags -- "-q" is redundant, and with no flags
it's doing "-u" which is the same as "-t" (with an added "-N").

collects/meta/drdr/metadata.rkt

@@ -10,15 +10,10 @@
   (define suffix (filename-extension a-path))
   (define default-cmd
     (and suffix
-         (cond
-           [(ormap (lambda (bs) (bytes=? suffix bs))
-                   (list #"ss" #"scm" #"scrbl" #"rkt" #"sls"))
-            '(racket "-qt" *)]
-           [(ormap (lambda (bs) (bytes=? suffix bs))
-                   (list #"rktl"))
-            '(racket "-f" *)]
-           [else
-            #f])))
+         (case (string->symbol (bytes->string/utf-8 suffix))
+           [(ss scm scrbl rkt sls) '(racket *)]
+           [(rktl)                 '(racket "-f" *)]
+           [else                   #f])))
   (define (replace-* s)
     (if (eq? '* s)
         (path->string* a-path)

collects/meta/props

@@ -531,9 +531,12 @@ path/s is either such a string or a list of them.
   ;; --------------------
   (Prop
    'drdr:command-line
-   (string-append
-    "command-line string (space-separated,\n    missing => default execution,"
-    " empty => no execution, \"~s\" => the file)")
+   (string-join
+    '("command-line string"
+      "space-separated with \"~s\" for the file, empty => no execution,"
+      "missing => use the default (\"racket ~s\" for *.rkt etc,"
+      "\"racket -f ~s\" for *.rktl)")
+    "\n      ")
    (lambda (str)
      (define (bad) (error "expecting an empty string, or one with `~s'"))
      (if (equal? str "")