Add a 'logpath' field to getrpcinfo

Summary:
The location of log files depends on the operating system and on which network you're using.
This provides a cross-platform way of finding the debug.log file.

Usage example:

tail -f `bitcoin-cli getrpcinfo | awk '/logpath/ {print $2}' | tr -d \"`

Backport of Core PR15483

Test Plan:

ninja && ./test/functional/test_runner.py interface_rpc
src/bitcoin-cli getrpcinfo

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7820