Improve util.py

Summary:
Make sure bitcoin.conf is handled as utf-8
Remove uses of ALL|FORKID as this is the default value for signrawtransaction

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1169