dkms: Enable debuginfo option to be set with zfs sysconfig file

On some Linux distributions, the kernel module build will not
default to building with debuginfo symbols, which can make it
difficult for debugging and testing.

For this case, we provide a flag to override the build to force
debuginfo to be produced for the kernel module build.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Neal Gompa <ngompa@datto.com>
Co-authored-by: Simon Watson <swatson@datto.com>
Signed-off-by: Neal Gompa <ngompa@datto.com>
Signed-off-by: Simon Watson <swatson@datto.com>
Closes #8304