* guix/pki.scm (signature-subject, signature-signed-data,
valid-signature?): New procedures.
* guix/scripts/authenticate.scm (guix-authenticate): Adjust to use
them.
* guix/pki.scm (ensure-acl): Make sure the directory of %ACL-FILE
exists.
* guix/scripts/archive.scm (generate-key-pair): Likewise for
%PUBLIC-KEY-FILE.
* guix/scripts/archive.scm (authorize-key): New procedure.
(guix-archive): Call it when OPTS contains 'authorize-key'.
* tests/guix-archive.sh: Add test with invalid public key.
* guix/pki.scm: Export '%acl-file'.
* doc/guix.texi (Invoking guix archive): Make it clear that '--import'
works only with authorized keys. Document '--authorize'.