Add/update copyright lines to top of seeder code

Summary:
Seeder files had either no copyright or only the first line of copyright
information and therefore needed updating.

Test Plan:

make

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4434