This is useful when people run "guix time-machine -C channels.scm",
where 'channels.scm' misses channel introductions.

* guix/channels.scm (%default-channel-url): New variable.
(%default-channels): Use it.
(ensure-default-introduction): New procedure.
(latest-channel-instance): Call it.

* .guix-channel: Add 'keyring-reference'.

This should come before patching, authentication, etc.

* guix/channels.scm (latest-channel-instance): Add #:validate-pull
parameter and honor it.  Return a single value: the instance.
(ensure-forward-channel-update): Change 'instance' parameter to 'commit'
and adjust accordingly.
(latest-channel-instances): Adjust to 'latest-channel-instance' changes.
* guix/scripts/pull.scm (warn-about-backward-updates): Change 'instance'
parameter to 'commit' and adjust accordingly.
* tests/channels.scm ("latest-channel-instances #:validate-pull"):
Likewise.

Fixes <https://bugs.gnu.org/22883>.

* guix/channels.scm (<channel>)[introduction]: New field.
(<channel-introduction>): New record type.
(%guix-channel-introduction): New variable.
(%default-channels): Use it.
(<channel-metadata>)[keyring-reference]: New field.
(%default-keyring-reference): New variable.
(read-channel-metadata, read-channel-metadata-from-source): Initialize
the 'keyring-reference' field.
(commit-short-id, verify-introductory-commit)
(authenticate-channel): New procedures.
(latest-channel-instance): Call 'authenticate-channel' when CHANNEL has
an introduction.
* tests/channels.scm (gpg+git-available?, commit-id-string): New
procedures.
("authenticate-channel, wrong first commit signer"):
("authenticate-channel, .guix-authorizations"): New tests.
* doc/guix.texi (Invoking guix pull): Mention authentication.

* tests/git-authenticate.scm (key-id): Remove.
(%ed25519-public-key-file, %ed25519-secret-key-file)
(%ed25519bis-public-key-file, %ed25519bis-secret-key-file)
(read-openpgp-packet, key-fingerprint): Move to...
* guix/tests/gnupg.scm: ... here.

* guix/git-authenticate.scm (authenticate-commits): Add #:keyring
parameter.

* guix/git-authenticate.scm (authenticated-commit-cache-file)
(cache-authenticated-commit, previously-authenticated-commits): Add
'key' parameter and honor it.
* build-aux/git-authenticate.scm (git-authenticate): Pass
"channels/guix" as the key.

* doc/contributing.texi (Building from Git): Move "make authenticate"
instructions below.

* guix/build-system/cmake.scm (cmake-build, cmake-cross-build): Add
substitutable keyword.

Fixes <http://issues.guix.gnu.org/41704

>.

* gnu/packages/package-management.scm (nix)[arguments]: Add
‘--sysconfdir=/etc’ configure flag and override it during installation.

Requested-and-tested-by: Peng Mei Yu <pengmeiyu@riseup.net>

* guix/scripts/pack.scm (%compressors): Add zstd.
* doc/guix.texi (Invoking guix pack): Document it.

* gnu/packages/emacs-xyz.scm (emacs-god-mode): Update to 2.17.0.

* gnu/packages/emulators.scm (bsnes)[inputs]: Remove gt+-2 and libx11.

* gnu/packages/crates-io.scm (rust-nom-5): New variable.
(rust-nom-4.2): Inherit from rust-nom-5.

* gnu/packages/crates-io.scm (rust-lexical-core-0.7): New variable.
(rust-lexical-core-0.4): Inherit from rust-lexical-core-0.7.

* gnu/packages/crates-io.scm (rust-static-assertions-1): New variable.
(rust-static-assertions-0.3): Inherit from rust-static-assertions-1.

* etc/committer.scm.in: New file.
* configure.ac: Configure it.

* gnu/packages/emulators.scm (bsnes): New variable.

* gnu/packages/cran.scm (r-xgboost): Update to 1.1.1.1.

* gnu/packages/cran.scm (r-genenet): Update to 1.2.15.

* gnu/packages/cran.scm (r-performance): Update to 0.4.7.

* gnu/packages/cran.scm (r-abn): Update to 2.2.1.
[propagated-inputs]: Add r-boot, r-brglm, r-entropy, and r-moments.

* gnu/packages/cran.scm (r-vioplot): Update to 0.3.5.
[native-inputs]: Add r-knitr.

* gnu/packages/cran.scm (r-circlize): Update to 0.4.10.

* gnu/packages/cran.scm (r-entropy): New variable.

* gnu/packages/cran.scm (r-brglm): New variable.

* gnu/packages/cran.scm (r-profilemodel): New variable.

An old snippet was causing the build to fail when using Emacs 27.

* gnu/packages/emacs-xyz.scm (emacs-yasnippet)[source]: Use git-file-name, and
remove obsolete snippet.
[phases]{home}: New phase.  This allows the rebindings test to pass.
{make-tests-writable, delete-rebinding-test}: Remove phases.
[description]: Re-indent.

* gnu/packages/engineering.scm (volk)[description]: Substitute
@acronym for the less appropriate @code.

* gnu/packages/engineering.scm (volk)[description]: Fix typo.

* gnu/packages/admin.scm (inxi-minimal): Update to 3.1.03-1.

* gnu/packages/build-tools.scm (tup): Update to 0.7.9.

Suggested by Maxim Cournoyer via IRC.

* doc/guix.texi (The Hurd in a Virtual Machine): Explicitly mention that
hurd-vm and childhurd are two names for this service.

* gnu/packages/gnome.scm (glade3)[arguments]: Enable tests. Add custom
pre-check phase.
[native-inputs]: Add hicolor-icon-theme and xorg-server-for-tests.

* guix/ui.scm (package->recutils): Use package-description-string instead of
package-description and P_.

* guix/ui.scm (package->recutils): package-synopsis-string already returns a
translated string. Do not attempt to translate it again.

* guix/ui.scm (relevance): When one of the regexps does not match, cut off
early and return 0. Do not try to match the remaining regexps.

* gnu/packages/ci.scm (cuirass): Update to 0.0.1-32.3db603c.

* guix/ci.scm (build-product?, build-product-id,
build-product-type, build-product-file-size,
build-product-path): Export them,
[id]: new field.

* guix/ci.scm (latest-builds): Add "status" option.