Add licensing and copyright information for all files in this repository. This
either happens in the file itself as a comment header or in the file
`.reuse/dep5`.
This commit also adds a Github workflow to check pull requests and adapts
copyright.pl to the changes.
Closes#8869
Also move timediff_t definitions from timeval.h to timediff.h and
then make timeval.h include the new standalone-capable timediff.h.
Reviewed-by: Jay Satiro
Reviewed-by: Daniel Stenberg
Supersedes #5888Closes#8595