download: Always use AI_ADDRCONFIG when resolving host names.

* guix/build/download.scm (open-socket-for-uri): Always pass
  AI_ADDRCONFIG to 'getaddrinfo' as recommended in the fine Guile
  manual.
* guix/ftp-client.scm (ftp-open): Ditto.

guix/build/download.scm

@@ -298,8 +298,8 @@ (define* (open-socket-for-uri uri-or-string #:key timeout)
 in seconds to wait for the connection to complete; passed TIMEOUT, an
 ETIMEDOUT error is raised."
   ;; Includes a fix for <http://bugs.gnu.org/15368> which affects Guile's
-  ;; 'open-socket-for-uri' up to 2.0.11 included, and uses 'connect*' instead
-  ;; of 'connect'.
+  ;; 'open-socket-for-uri' up to 2.0.11 included, uses 'connect*' instead
+  ;; of 'connect', and uses AI_ADDRCONFIG.
 
   (define http-proxy (current-http-proxy))
   (define uri (ensure-uri (or http-proxy uri-or-string)))
@@ -309,9 +309,9 @@ (define addresses
        (getaddrinfo (uri-host uri)
                     (cond (port => number->string)
                           (else (symbol->string (uri-scheme uri))))
-                    (if port
-                        AI_NUMERICSERV
-                        0))
+                    (if (number? port)
+                        (logior AI_ADDRCONFIG AI_NUMERICSERV)
+                        AI_ADDRCONFIG))
        (lambda (ai1 ai2)
          (equal? (addrinfo:addr ai1) (addrinfo:addr ai2))))))
 

guix/ftp-client.scm

@@ -134,7 +134,9 @@ (define* (ftp-open host #:optional (port 21) #:key timeout)
   (define addresses
     (getaddrinfo host
                  (if (number? port) (number->string port) port)
-                 (if (number? port) AI_NUMERICSERV 0)))
+                 (if (number? port)
+                     (logior AI_ADDRCONFIG AI_NUMERICSERV)
+                     AI_ADDRCONFIG)))
 
   (let loop ((addresses addresses))
     (let* ((ai (car addresses))