pki: Introduce 'write-acl', and fix wrong conversion in 'ensure-acl'.

* 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'.

pki: Keep ACL in native sexp format to speed up 'authorized-key?'.

* 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.

pki: Add 'signature-case' macro.

* guix/pki.scm (%signature-status): New procedure.
  (signature-case): New macro.
* tests/pki.scm (%secret-key, %alternate-secret-key): New variables.
  ("signature-case valid-signature", "signature-case invalid-signature",
  "signature-case hash-mismatch", "signature-case unauthorized-key",
  "signature-case corrupt-signature"): New tests.

Merge branch 'master' into core-updates

pki: Factorize signature manipulation procedures.

* guix/pki.scm (signature-subject, signature-signed-data,
  valid-signature?): New procedures.
* guix/scripts/authenticate.scm (guix-authenticate): Adjust to use
  them.

Merge branch 'master' into core-updates

archive: Make sure $sysconfdir/guix exists.

* 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.

archive: Add '--authorize'.

* 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'.

archive: Add '--generate-key'.

* guix/pk-crypto.scm (error-source, error-string): New procedures.
* guix/pki.scm (%private-key-file): New variable.
* guix/scripts/archive.scm (show-help): Document '--generate-key'.
  (%options): Add "generate-key".
  (generate-key-pair): New procedure.
  (guix-archive): Call 'generate-key' when OPTS contains a
  'generate-key' pair.
* doc/guix.texi (Setting Up the Daemon): Suggest generating a key pair.
  (Invoking guix archive): Document '--generate-key'.

Add (guix pki).

* guix/pki.scm, tests/pki.scm: New files.
* Makefile.am (MODULES): Add 'guix/pki.scm'.
  (SCM_TESTS): Add 'tests/pki.scm'.