Fix feedback in 'sweeprolog-pack-install'

Fix feedback in 'sweeprolog-pack-install'

Have 'sweeprolog-pack-install' actually report its result, instead of
reporting failure even when successful.

* sweep.pl (sweep_pack_install/2): Bind the output argument to non-nil
on success.
* sweeprolog.el (sweeprolog-pack-description-max-width): New user
option.
(sweeprolog-read-pack-name): Use display text properties for alignment
instead of repeated spaces.