New -includeconf argument for including external configuration files

Summary:

• adds -includeconf=<path>, where <path> is relative to datadir or to

the path of the file being read, if in a file

• protects against circular includes
• updates help docs

Backport of core PR10267
https://github.com/bitcoin/bitcoin/pull/10267/files

Includes a fix from PR13126
https://github.com/bitcoin/bitcoin/pull/10267#issuecomment-388580967

Completes T541
Progresses towards T652

Test Plan:

make check
./test/functional/test_runner.py feature_includeconf

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Maniphest Tasks: T541

Differential Revision: https://reviews.bitcoinabc.org/D3035