Janitorial work: add copyright headers.

This is supposed to be a step towards addressing issue #2833
and applying GNU guidelines more consistently;

• Add copyright headers to every non-trivial file longer than
  a dozen LOC.

• The three sort of files I’m not touching (for now at least) are:

  • Makefiles (*/GNUmakefile, make/ and stepmake/)
  • Documentation/*.tely, *.texi, snippets/ etc.
  • input/regression/*

I’m not quite sure which of these really require copyright headers,
so that may need to be discussed.