gnu: Add ntp

* gnu/packages/ntp.scm: New file.
* gnu-system.am (gnu/packages/ntp.scm): Add it.

emacs: Improve interface for working with multiple profiles.

Suggested by David Thompson, Ludovic Courtès and Mathieu Lirzin.

* emacs/guix-base.el (guix-profile-prompt): New procedure.
  (guix-set-current-profile): Use it.
  (guix-buffer-name-simple, guix-buffer-name-default, guix-buffer-name): New
  procedures.
  (guix-buffer-name-function, guix-profile): New variables.
  (guix-set-vars, guix-get-entries, guix-get-show-entries, guix-set-buffer,
  guix-history-call, guix-process-package-actions,
  guix-continue-package-operation-p, guix-delete-generations,
  guix-switch-to-generation): Add 'profile' argument.
* emacs/guix.el (guix-get-show-packages, guix-get-show-generations,
  guix-search-by-name, guix-search-by-regexp, guix-installed-packages,
  guix-obsolete-packages, guix-all-available-packages,
  guix-newest-available-packages, guix-generations, guix-generations-by-time):
  Likewise.
  (guix-last-generations): New command.
* emacs/guix-info.el: Adjust for using 'profile' argument where needed.
* emacs/guix-list.el: Likewise.
* doc/emacs.texi (Emacs Commands): Document 'guix-last-generations' and using
  "C-u" for commands.
  (Emacs Buffer Names): Document 'guix-buffer-name-function'.

packages: Add 'package-transitive-supported-systems'.

* guix/packages.scm (package-transitive-supported-systems): New procedure.
* tests/packages.scm ("package-transitive-supported-systems"): New test.
* build-aux/hydra/gnu-system.scm (package->job): Use it.

gnu: valgrind: Don't build on mipsel64-linux.

* gnu/packages/valgrind.scm (valgrind): Add 'supported-systems'.

gnu: sdl-gfx: Build only on Intel machines.

* gnu/packages/sdl.scm (sdl-gfx): Add 'supported-systems' field.

derivations: 'graft-derivation' correctly handles multiple-output sources.

* guix/derivations.scm (graft-derivation): Add DRV as input for each one
  of OUTPUT-NAMES.

packages: Improve the package-to-derivation cache.

* guix/packages.scm (cache): Preserve the former values associated with
  PACKAGE.
  (cached): Bind SYSTEM in 'let', to prevent double evaluation.

derivations: Add #:system parameter to 'graft-derivation'.

* guix/derivations.scm (graft-derivation): Add #:system parameter.

derivations: Introduce 'graft' record type.

* guix/derivations.scm (<graft>): New record type.
  (graft-derivation): Rename 'replacements' to 'grafts', and expect it
  to be a list of <graft> records.  Adjust accordingly.
* tests/derivations.scm ("graft-derivation"): Use 'graft' instead of
  pairs in argument to 'graft-derivation'.

records: Improve the docstring of 'define-record-type*'.

* guix/records.scm (define-record-type*): Agument the docstring, give
  examples.

doc: Replace "emacs" with "Emacs" in node names.

Suggested by Ludovic Courtès.

* doc/emacs.texi: Use "Emacs" in node names.

emacs: Add 'guix-generations-by-time' command.

* emacs/guix-main.scm (find-generations): Add finding generations by time.
* emacs/guix-utils.el (guix-read-date): New procedure.
* emacs/guix-base.el (guix-messages): Add new messages.
* emacs/guix.el (guix-generations-by-time): New command.
* doc/emacs.texi (emacs Commands): Document it.

download: Allow use of substitutes.

See <https://bugs.gnu.org/18747> for the original report.

* guix/download.scm (url-fetch): Comment out #:local-build? argument.
* guix/git-download.scm (git-fetch): Likewise.
* guix/svn-download.scm (svn-fetch): Likewise.

gnu: linux-libre: Update to 3.17.1.

* gnu/packages/linux.scm (linux-libre): Update to version 3.17.1.

emacs: Support auto-updating after performing REPL operation.

* emacs/guix-backend.el (guix-operation-buffer): New variable.
  (guix-eval-in-repl): Use it.  Add optional 'operation-buffer' argument.
* emacs/guix-base.el (guix-update-after-operation): New variable.
  (guix-switch-to-buffer, guix-list-or-info-buffer-p, guix-buffers,
  guix-update-buffers-maybe): New procedures.
  (guix-set-buffer): Use 'guix-switch-to-buffer'.  Add optional 'no-display'
  argument.
  (guix-process-package-actions, guix-delete-generations,
  guix-switch-to-generation): Add optional 'operation-buffer' argument.
* emacs/guix-info.el (guix-package-info-insert-action-button,
  guix-generation-info-insert-number, guix-generation-info-insert-current):
  Adjust for using operation buffer.
* emacs/guix-list.el (guix-list-execute-package-actions,
  guix-generation-list-switch, guix-generation-list-execute): Likewise.

emacs: Add hooks for REPL operations.

* emacs/guix-backend.el (guix-before-repl-operation-hook,
  guix-after-repl-operation-hook, guix-repl-operation-p): New variables.
  (guix-repl-operation-success-message, guix-repl-output-filter): New
  procedures.
  (guix-start-repl): Adjust for using 'guix-repl-output-filter'.
  (guix-eval-in-repl): Honor 'guix-before-repl-operation-hook'.

guix lint: add the --checkers option.

* guix/scripts/lint.scm: add the "--checkers" option.
* doc/guix.texi (Invoking guix lint): Document it.
* tests/guix-lint.sh: New file
* Makefile.am (SCM_TESTS): Add it.

gnu: gnutls: Patch bug in 'set-session-server-name!'.

* gnu/packages/patches/gnutls-server-name-fix.patch: New file.
* gnu/packages/gnutls.scm (gnutls)[source]: Use it.
* gnu-system.am (dist_patch_DATA): Add it.

gnu: Add xboing

* gnu/packages/games.scm (xboing): New variable.

utils: Micro-optimize 'bytevector->base16-string'.

This reduces run time of (package-derivation s emacs) by ~1.5%.

* guix/utils.scm (bytevector->base16-string): Traverse BV from the end
  so as to use 'string-concatenate' instead of
  'string-concatenate-reverse'.