guix system: Add 'search' command.

* guix/scripts/system.scm (resolve-subcommand): New procedure.
(process-command): Handle 'search'.
(guix-system): Likewise.
(show-help): Augment.
* guix/scripts/system/search.scm: New file.
* po/guix/POTFILES.in: Add it.
* Makefile.am (MODULES): Add it.
* guix/ui.scm (%text-width): Export.
* doc/guix.texi (Invoking guix system): Document it.
(Service Types and Services): Mention 'guix system search'.
* tests/guix-system.sh: Test it.