channels: Make channel introductions public.

* guix/channels.scm (<channel-introduction>): Rename constructor to
'%make-channel-introduction'.
(make-channel-introduction): New procedure.
* tests/channels.scm ("authenticate-channel, wrong first commit signer")
("authenticate-channel, .guix-authorizations"): Use
'make-channel-introduction' without '@@' and without third argument.
* doc/guix.texi (Channels)[Channel Authentication, Specifying Channel
Authorizations]: New subsections.

guix/channels.scm

@@ -69,7 +69,9 @@
             channel-location
 
             channel-introduction?
-            ;; <channel-introduction> accessors purposefully omitted for now.
+            make-channel-introduction
+            channel-introduction-first-signed-commit
+            channel-introduction-first-commit-signer
 
             openpgp-fingerprint->bytevector
             openpgp-fingerprint
@@ -130,13 +132,19 @@
 ;; commit so that only them may emit this introduction.  Introductions are
 ;; used to bootstrap trust in a channel.
 (define-record-type <channel-introduction>
-  (make-channel-introduction first-signed-commit first-commit-signer
-                             signature)
+  (%make-channel-introduction first-signed-commit first-commit-signer
+                              signature)
   channel-introduction?
   (first-signed-commit  channel-introduction-first-signed-commit) ;hex string
   (first-commit-signer  channel-introduction-first-commit-signer) ;bytevector
   (signature            channel-introduction-signature))          ;string
 
+(define (make-channel-introduction commit signer)
+  "Return a new channel introduction: COMMIT is the introductory where
+authentication starts, and SIGNER is the OpenPGP fingerprint (a bytevector) of
+the signer of that commit."
+  (%make-channel-introduction commit signer #f))
+
 (define (openpgp-fingerprint->bytevector str)
   "Convert STR, an OpenPGP fingerprint (hexadecimal string with whitespace),
 to the corresponding bytevector."