Documents improvements and small fixes

• Fix installation of /etc/kernel/install.d/dkms and /etc/dkms/sign_helper.sh script.
• Clean up Makefile, docs and man page.
• Be more consistent with versioning, fix creation of tarballs, properly expand man page variables.
• Use more condensed output for commands, print when invoking the signing hook.
• Add documentation for enabling signing of modules and enrolling MOK certificates.