* guix/gnupg.scm (%gpgv-command, current-keyring): New variables
(gnupg-verify): Add optional 'keyring' parameter. Use 'gpgv' instead of
'gpg' and pass it '--keyring'.
(gnupg-receive-keys): Add optional 'keyring' parameter and honor it.
(gnupg-verify*): Add #:keyring and honor it.
* guix/scripts/refresh.scm (%options, show-help): Add '--keyring'.
(guix-refresh): Parameterize CURRENT-KEYRING.
* doc/guix.texi (Invoking guix refresh): Document '--keyring' and the
keybox format.
* guix/gnu-maintenance.scm (download-tarball): Add a 'key-download'
keyword argument and pass it to 'gnupg-verify*'. Make
'archive-type' a keyword argument.
(package-update): Add a 'key-download' keyword argument. Pass
'archive-type' and 'key-download' keyword arguments to
'download-tarball'.
* guix/gnupg.scm: Import (ice-9 i18n) and (guix ui).
(gnupg-verify*): Add a 'key-download' keyword argument and adjust
'gnupg-verify*' to use it. Make 'server' a keyword argument.
* guix/scripts/refresh.scm (show-help, %options): Add and document
'--key-download'.
(update-package): Add a 'key-download' keyword argument and pass it
to 'package-update'.
(guix-refresh): Pass 'key-download' to 'update-package'. Limit
lines to a maximum of 79 characters.
* guix/gnupg.scm (%gpg-command): Turn into a SRFI-39 parameter.
(%openpgp-key-server): Likewise. Default to pgp.mit.edu, as
keys.gnupg.net is unreliable.
Update users.
