Small fixes

• Install the new kernel_install.d_dkms and sign_helper.sh scripts in all targets under /etc.
• Do not require root access for the build command as described in the manual page.
• Fix support for /etc/sysconfig/kernel directory.
• Improve test dkms.conf file.
• Add support for zstd compressed modules.
• Document NO_WEAK_MODULES in the man page.
• Use kmod as a package dependency on Debian when available.
• Be more verbose when failing due to missing module.