Draft: Migrate licenses in dbus source to SPDX format

With the commits from this merge request, the dbus source code will have license information based on the SPDX notation. Existing copyright information also uses SPDX keywords, as this reduces the effort required to capture corresponding information.

Reuse conformance can be checked, for example, with sudo docker run --rm --volume $(pwd):/data fsfe/reuse lint, which showed no missing license information in (c/cpp) source files after applying the patches from this merge request.

A question that is often asked in this context is whether the full license text must be included in each file. The Linux kernel developers say on this:

The common way of expressing the license of a source file is to add the
matching boilerplate text into the top comment of the file.  Due to
formatting, typos etc. these "boilerplates" are hard to validate for
tools which are used in the context of license compliance.

An alternative to boilerplate text is the use of Software Package Data
Exchange (SPDX) license identifiers in each source file.  SPDX license
identifiers are machine parsable and precise shorthands for the license
under which the content of the file is contributed.  SPDX license
identifiers are managed by the SPDX Workgroup at the Linux Foundation and
have been agreed on by partners throughout the industry, tool vendors, and
legal teams.  For further information see https://spdx.org/

This is important because especially with (L)GPL there is a stricter condition for this and the Linux kernel mainly uses these licenses.

resolves #394

depends on !378 (merged)