status: Don't display download URLs for '--verbosity=1'.

With this change, each substitute occupies a single line of
output (instead of two) when using '-v1', the default for 'guix package'
& co.

* guix/status.scm (print-build-event): Add #:print-urls? and honor it.
(print-build-event/quiet): Pass #:print-urls? #f.
(print-build-event/quiet-with-urls): New procedure.
(logger-for-level): Add case for LEVEL 2.
* doc/guix.texi (Common Build Options): Adjust '--verbosity'
documentation.