* guix/scripts/refresh.scm (%options): Add `--key-server' and `--gpg'.
(show-help): Update accordingly.
(update-package): New procedure, formerly in `guix-refresh'.
(guix-refresh): Use it. Parameterize `%openpgp-key-server' and
`%gpg-command'.
* guix/scripts/refresh.scm (guix-refresh): When updating a source file,
gracefully handle the case where TARBALL is #f.
Reported by Nikita Karetnikov <nikita@karetnikov.org>.
* guix/scripts/refresh.scm (%options): Add `--select'.
(show-help): Likewise. Augment initial help text.
(guix-refresh)[core-package?]: New procedure.
Use it when selecting packages.