Accept cash magic in addition to regular magic.

Summary: This will allow us to transition toward using a new magic once enough of the network supports it.

Test Plan:
Updated the regression test mininode implementation to use the new magic.

../qa/pull-tester/rpc-tests.py -extended

Reviewers: freetrader, zander, #bitcoin_abc, CCulianu

Reviewed By: zander, #bitcoin_abc, CCulianu

Subscribers: CCulianu

Differential Revision: https://reviews.bitcoinabc.org/D400