Improve licensing

[mafik]

MIT license (current one) is not great because:

1. Users can be spied on by customized Gatekeeper instances (Affero license should help with that)
2. Multiple copyright holders make license enforcement difficult (copyright assignment should address that)

Additionally some cleanup tasks should be done:

1. Licenses of dependencies (libsystemd) should be included in some clear fashion (maybe NOTICES file)
2. All files should include "SPDX-License-Identifier"
3. The new licensing model should be explained in some central doc

[mafik]

Also AUTHORS file: https://opensource.google/documentation/reference/releasing/authors

Copyright assignment might be difficult. It seems it requires some paperwork. See:

• https://opensource.google/documentation/reference/cla
• https://www.gnu.org/licenses/why-assign.en.html
• https://en.wikipedia.org/wiki/Contributor_License_Agreement