Add support for slackware distribution (linux_install.sh) (#4369)

* Add support for slackware distribution (linux_install.sh)

The required packages are not provided by the official repository. The packages
need to be installed from slackbuilds.org either manually or with the help of
third party tools like sbotools.

* Modify linux_install.sh according to code review

Modifies the Slackware section of the linux_install.sh script based on the
suggestions from the code review.

* Modify utils/linux_install.sh according to suggestions