guix refresh: Keep only the newest versions of packages as upgrade candidates.

* guix/scripts/refresh.scm (guix-refresh)[keep-newest]: New procedure.
  Use it to keep only once copy of each package.

Use `port-sha256' and `open-sha256-port'.

* guix/derivations.scm (derivation-hash): Add comment as to why we keep
  the plain `sha256' call.
* guix/scripts/download.scm (guix-download): Use `port-sha256' instead
  of (compose sha256 get-bytevector-all).
* guix/scripts/refresh.scm (update-package): Likewise.

Move `sha256' to (guix hash).

* guix/utils.scm (sha256): Move to...
* guix/hash.scm: ... here.  New file.
* Makefile.am (MODULES): Add it.
* guix/derivations.scm, guix/scripts/download.scm,
  guix/scripts/hash.scm, guix/scripts/refresh.scm, tests/base32.scm,
  tests/derivations.scm, tests/store.scm: Use (guix hash).

guix refresh: Add '--key-download'.

* 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.

refresh: Add `--key-server' and `--gpg'.

* 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'.

refresh: Gracefully handle failure to download a tarball.

* 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>.

refresh: Change default to not update source files.

* guix/scripts/refresh.scm (%options): Change `--dry-run' to `--update'.
  (show-help): Adjust accordingly.
  (guix-refresh): Likewise.

ui: Add `args-fold*' and use it.

* guix/ui.scm (args-fold*): New procedure.
* guix/scripts/build.scm, guix/scripts/download.scm,
  guix/scripts/gc.scm, guix/scripts/hash.scm, guix/scripts/import.scm,
  guix/scripts/package.scm, guix/scripts/pull.scm,
  guix/scripts/refresh.scm: Use `args-fold*' instead of `args-fold'.

Merge branch 'master' into core-updates

Conflicts:
	Makefile.am
	gnu/packages/autotools.scm
	gnu/packages/guile.scm
	gnu/packages/python.scm
	gnu/packages/shishi.scm
	guix/gnu-maintenance.scm
	guix/scripts/build.scm
	guix/scripts/gc.scm
	guix/scripts/package.scm
	guix/scripts/substitute-binary.scm
	guix/ui.scm
	nix/nix-daemon/guix-daemon.cc
	test-env.in
	tests/nar.scm
	tests/store.scm

refresh: Add `--select'.

* 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.

Add `guix refresh' and related auto-update tools.

* guix/gnu-maintenance.scm (ftp-server/directory)[quirks]: Add glib.
  (package-update-path, download-tarball, package-update,
  update-package-source): New procedures.
* guix/gnupg.scm, guix/scripts/refresh.scm: New files.
* Makefile.am (MODULES): Add them.
* guix/utils.scm (file-extension): New procedure.