Update copyright and license
meta/autofix/fixlegal.py: ensure correct notices

Make sure that legal notices are present and correct in the relevant
files by parsing all files and updating or inserting notices as
appropriate.

Discovery of file creation date from Git is quite expensive so
optionally only check files modified since 'origin/master'.

Retire outdated copyright check in favor of the new mechanism.

Test Plan:
$ ./precommit.sh
inspect modified files (submitted in previous commit),
scan list of unmodified files for any accidental omissions.