Refactor the whole SIGHASH_FORKID mechanism.

Summary: There was some code repeatition and overall, the structure could be improved.

Test Plan:

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

Updated test according to code changes.

Reviewers: freetrader, #bitcoin_abc, kyuupichan

Reviewed By: #bitcoin_abc, kyuupichan

Subscribers: kyuupichan

Differential Revision: https://reviews.bitcoinabc.org/D341