Return a file size string when possible.

This takes the advice from
  http://files.stairways.com/other/ftp-list-specs-info.txt
further: search for the date by an explicit occurrence of a known month
name.  This means that we won't see files with bad names (they'd be
filtered out of the result), but the filtered out entries are ones that
would not be usable with `ftp-make-file-seconds'.

When the month is found, and the entry is a file, look for a number
preceding the month, and if found, return it as the file size string.
This is a minor change in the API.  (But it's probably better to either
revise it further, or eventually make it irrelevant by exposing the
interesting functionality via `net/url'.)

original commit: 6a1336e75edff75b7fc8a22a31d776acadc536eb

collects/net/ftp-unit.rkt

@@ -159,7 +159,9 @@
                       #"250" void (void)))
 
 (define re:dir-line
-  #rx"^(.)......... .* ([A-Z].* .* [0-9][0-9]:?[0-9][0-9]) (.*)$")
+  (regexp (string-append
+           "^(.)(.*) ((?i:jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)"
+           " .* [0-9][0-9]:?[0-9][0-9]) (.*)$")))
 
 (define (ftp-directory-list tcp-ports)
   (define tcp-data (establish-data-connection tcp-ports))
@@ -173,9 +175,15 @@
                       (ftp-connection-out tcp-ports)
                       #"226" print-msg (void))
   (for*/list ([l (in-list lines)]
-              [m (in-value (regexp-match re:dir-line l))]
+              [m (in-value (cond [(regexp-match re:dir-line l) => cdr]
+                                 [else #f]))]
               #:when m)
-    (cdr m)))
+    (define size (cond [(and (equal? "-" (car m))
+                             (regexp-match #rx"([0-9]+) *$" (cadr m)))
+                        => cadr]
+                       [else #f]))
+    (define r `(,(car m) ,@(cddr m)))
+    (if size `(,@r ,size) r)))
 
 (define (ftp-download-file tcp-ports folder filename)
   ;; Save the file under the name tmp.file, rename it once download is