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.

guix/scripts/system.scm

@@ -73,7 +73,6 @@
   "Read the operating-system declaration from FILE and return it."
   (load* file %user-module))
 
-
 
 ;;;
 ;;; Installation.
@@ -751,6 +750,8 @@ Some ACTIONS support additional ARGS.\n"))
   (newline)
   (display (G_ "The valid values for ACTION are:\n"))
   (newline)
+  (display (G_ "\
+   search           search for existing service types\n"))
   (display (G_ "\
    reconfigure      switch to a new operating system configuration\n"))
   (display (G_ "\
@@ -937,6 +938,12 @@ resulting from command-line parsing."
                              #:gc-root (assoc-ref opts 'gc-root)))))
         #:system system))))
 
+(define (resolve-subcommand name)
+  (let ((module (resolve-interface
+                 `(guix scripts system ,(string->symbol name))))
+        (proc (string->symbol (string-append "guix-system-" name))))
+    (module-ref module proc)))
+
 (define (process-command command args opts)
   "Process COMMAND, one of the 'guix system' sub-commands.  ARGS is its
 argument list and OPTS is the option alist."
@@ -949,6 +956,8 @@ argument list and OPTS is the option alist."
                       ((pattern) pattern)
                       (x (leave (G_ "wrong number of arguments~%"))))))
        (list-generations pattern)))
+    ((search)
+     (apply (resolve-subcommand "search") args))
     ;; The following commands need to use the store, but they do not need an
     ;; operating system configuration file.
     ((switch-generation)
@@ -978,7 +987,7 @@ argument list and OPTS is the option alist."
           (case action
             ((build container vm vm-image disk-image reconfigure init
               extension-graph shepherd-graph list-generations roll-back
-              switch-generation)
+              switch-generation search)
              (alist-cons 'action action result))
             (else (leave (G_ "~a: unknown action~%") action))))))