processes: Allow 'less' to properly estimate line length.

Until now, the first few lines in the output of 'guix processes' could
disappear in 'less'.

* guix/ui.scm (call-with-paginated-output-port): Add #:less-options
parameter and honor it.
(with-paginated-output-port): Allow callers to pass #:less-options.
* guix/scripts/processes.scm (guix-processes): Pass #:less-options to
'with-paginated-output-port'.