* guix/pki.scm (write-acl): New procedure.
(ensure-acl): Use it. Fixes a regression introduced in 39831f1,
whereby 'ensure-acl' would yield a wrong-type-arg error.
* guix/scripts/archive.scm (authorize-key): Use 'write-acl'.
* guix/pki.scm (acl-entry-sexp, acl-sexp): Remove.
(public-keys->acl, current-acl): Return a native sexp.
(acl->public-keys, authorized-key?): Expect ACL to be a native sexp.
* guix/scripts/archive.scm (authorize-key): Convert ACL to
canonical-sexp when writing it.
* 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'.