store: Define a default port for TCP connections.

* guix/store.scm (%default-guix-port): New variable.
(connect-to-daemon)[connect]: Use it when (uri-port uri) is #f.
* doc/guix.texi (The Store): Mention the default port number.

doc/guix.texi

@@ -3770,8 +3770,8 @@ These are for Unix-domain sockets.
 
 @item guix
 These URIs denote connections over TCP/IP, without encryption nor
-authentication of the remote host.  The URI must always specify both the
-host name and port number:
+authentication of the remote host.  The URI must specify the host name
+and optionally a port number (by default port 44146 is used):
 
 @example
 guix://master.guix.example.org:1234

guix/store.scm

@@ -383,6 +383,10 @@
       (connect s a)
       s)))
 
+(define %default-guix-port
+  ;; Default port when connecting to a daemon over TCP/IP.
+  44146)
+
 (define (open-inet-socket host port)
   "Connect to the Unix-domain socket at HOST:PORT and return it.  Raise a
 '&nix-connection-error' upon error."
@@ -446,12 +450,8 @@ name."
             (open-unix-domain-socket (uri-path uri))))
          ('guix
           (lambda (_)
-            (unless (uri-port uri)
-              (raise (condition (&nix-connection-error
-                                 (file (uri->string uri))
-                                 (errno EBADR))))) ;bah!
-
-            (open-inet-socket (uri-host uri) (uri-port uri))))
+            (open-inet-socket (uri-host uri)
+                              (or (uri-port uri) %default-guix-port))))
          ((? symbol? scheme)
           ;; Try to dynamically load a module for SCHEME.
           ;; XXX: Errors are swallowed.