Add missing copyright/disclaimer headers

No changes to core code, just commented entries. This adds a disclaimer
to the existing files in `textpattern/update` because they make changes
and don’t have an existing disclaimer.

Stuff in `rpc` is still credited to @kusor with appropriate author and
documentation links - again, no changes to code, just some additional
information and legal coverage.

IANAL.
Merge pull request #447 from petecooper/copyright-header-tidy

Add missing copyright/disclaimer headers