* guix/scripts/pull.scm (display-profile-news): Return true when there's
more to display.
(display-news-entry-title): New procedure.
(display-news-entry): Use it.
(display-channel-specific-news): Return true when there's more to
display.
(display-channel-news-headlines): New procedure.
(build-and-install): Call it. When 'display-channel-news-headlines' or
'display-profile-news' returns #t, print a hint to run "pull --news".
(display-new/upgraded-packages): Return true when there's more to display.