scripts: Use 'define-command' and have 'guix help' use that.

scripts: Use 'define-command' and have 'guix help' use that.

This changes 'guix help' to print a short synopsis for each command and
to group commands by category.

* guix/scripts.scm (synopsis, category): New variables.
(define-command-categories, define-command): New macros.
(%command-categories): New variable.
* guix/ui.scm (<command>): New record type.
(source-file-command): New procedure.
(command-files): Return absolute file names.
(commands): Return a list of <command> records.
(show-guix-help)[display-commands, category-predicate]: New procedures.
Display commands grouped in three categories.
* guix/scripts/archive.scm (guix-archive): Use 'define-command'.
* guix/scripts/authenticate.scm (guix-authenticate): Likewise.
* guix/scripts/build.scm (guix-build): Likewise.
* guix/scripts/challenge.scm (guix-challenge): Likewise.
* guix/scripts/container.scm (guix-container): Likewise.
* guix/scripts/copy.scm (guix-copy): Likewise.
* guix/scripts/deploy.scm (guix-deploy): Likewise.
* guix/scripts/describe.scm (guix-describe): Likewise.
* guix/scripts/download.scm (guix-download): Likewise.
* guix/scripts/edit.scm (guix-edit): Likewise.
* guix/scripts/environment.scm (guix-environment): Likewise.
* guix/scripts/gc.scm (guix-gc): Likewise.
* guix/scripts/git.scm (guix-git): Likewise.
* guix/scripts/graph.scm (guix-graph): Likewise.
* guix/scripts/hash.scm (guix-hash): Likewise.
* guix/scripts/import.scm (guix-import): Likewise.
* guix/scripts/install.scm (guix-install): Likewise.
* guix/scripts/lint.scm (guix-lint): Likewise.
* guix/scripts/offload.scm (guix-offload): Likewise.
* guix/scripts/pack.scm (guix-pack): Likewise.
* guix/scripts/package.scm (guix-package): Likewise.
* guix/scripts/perform-download.scm (guix-perform-download): Likewise.
* guix/scripts/processes.scm (guix-processes): Likewise.
* guix/scripts/publish.scm (guix-publish): Likewise.
* guix/scripts/pull.scm (guix-pull): Likewise.
* guix/scripts/refresh.scm (guix-refresh): Likewise.
* guix/scripts/remove.scm (guix-remove): Likewise.
* guix/scripts/repl.scm (guix-repl): Likewise.
* guix/scripts/search.scm (guix-search): Likewise.
* guix/scripts/show.scm (guix-show): Likewise.
* guix/scripts/size.scm (guix-size): Likewise.
* guix/scripts/substitute.scm (guix-substitute): Likewise.
* guix/scripts/system.scm (guix-system): Likewise.
* guix/scripts/time-machine.scm (guix-time-machine): Likewise.
* guix/scripts/upgrade.scm (guix-upgrade): Likewise.
* guix/scripts/weather.scm (guix-weather): Likewise.