Support Debian DKMS builds

Support Debian DKMS builds

scripts/dkms.mkconf calls configure with
`--with-linux=${kernel_source_dir}`, but Debian puts it kernel source at
`/lib/modules/<version>/source`. This patch adds the same logic to the
DKMS file produced by `scripts/dkms.mkconf` that Debian has shipped in
its official ZFS packaging: at DKMS build time, it checks if the system
is a Debian system, and adjusts the path accordingly.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #7358
Closes #7540
Closes #7554