[backport#15864] Fix datadir handling

Summary:
Merge #15864: Fix datadir handling

ffea41f5301d5582665cf10ba5c2b9547a1443de Enable all tests in feature_config_args.py (Hennadii Stepanov) (N.f.B. backported in D5807)
66f5c17f8a3fe06fc65191e379ffc04e43cbbc86 Use CheckDataDirOption() for code uniformity (Hennadii Stepanov)
7e33a18a34b1a9b0f115076c142661d6d30c0585 Fix datadir handling in bitcoin-cli (Hennadii Stepanov)
b28dada37465c0a773cf08b0e6766f0081bcb943 Fix datadir handling in bitcoin-qt (Hennadii Stepanov)
50824093bb2d68fe1393dfd636fab5f8795faa5d Fix datadir handling in bitcoind (Hennadii Stepanov)
740d41ce9f7fdf209366e930bd0fdcc6b1bc6b79 Add CheckDataDirOption() function (Hennadii Stepanov)
c1f325126cf51d28dce8da74bfdf5cd05ab237ea Return absolute path early in AbsPathForConfigVal (Hennadii Stepanov)

Pull request description:

Fix #15240, see: https://github.com/bitcoin/bitcoin/issues/15240#issuecomment-487353760
Fix #15745
Fix broken feature_config_args.py tests (disabled by MarcoFalke@fabe28a0cdcfa13e0e595a0905e3642a960d3077). All test are enabled now.
This PR is alternative to #13621.

User's $HOME directory is not touched unnecessarily now.

~To make reviewing easier only bitcoind code is modified (neither bitcoin-cli nor bitcoin-qt).~

Refs:

• https://github.com/bitcoin/bitcoin/issues/15745#issuecomment-479852569 by laanwj
• #16220

Depends on D5807

This is a partial backport of Core PR15864

Test Plan:

ninja check check-functional

Reviewers: deadalnix, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5812