Update user agent filter in makeseeds.py

Summary: Remove BCash, and add BCHD and BU

Test Plan:
I created a seeds_main.txt with different variants of valid and
invalid user agent strings and version for the different implementations.
Then ran python3 makeseeds.py < seeds_main.txt > nodes_main.tx and checked
that the appropriate IPs were selected.

Reviewers: deadalnix, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4319