src: use SPDX identifiers

The SPDX identifiers [1] are a legally binding shorthand, which can be
used instead of the full boiler plate text. This is another step towards
implementing the full REUSE spec [2] to help with copyright and
licensing audits and compliance.

This will reduce a lot a manual work required for the licensing audit
required in Debian on each update.

[1] https://spdx.org/ids-how
[2] https://reuse.software/tutorial/

Change-Id: Ia28ed8c14984ac9acd140ef544fd6e09b96fb03b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>