libmd: Add SPDX-License-Identifier tags

[tuukka.pasanen_ilmi.fi]

Summary

This is probably BSD-3-Clause license but as it has been ported from Golang and go itself it should be checked that it's correct.

Current license for this should be this one: LICENSE

Some BSD-3-Clause files under lib/libmd were missing the SPDX-License-Identifier-tag.

Sponsored by: The FreeBSD Foundation