Merge #15864: Fix datadir handling

ffea41f530 Enable all tests in feature_config_args.py (Hennadii Stepanov)
66f5c17f8a Use CheckDataDirOption() for code uniformity (Hennadii Stepanov)
7e33a18a34 Fix datadir handling in bitcoin-cli (Hennadii Stepanov)
b28dada374 Fix datadir handling in bitcoin-qt (Hennadii Stepanov)
50824093bb Fix datadir handling in bitcoind (Hennadii Stepanov)
740d41ce9f Add CheckDataDirOption() function (Hennadii Stepanov)
c1f325126c 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

Top commit has no ACKs.

Tree-SHA512: 4a4cda10e0b67c8f374da0c9567003d2b566d948e7f8550fe246868b5794c15010e88ea206009480b9cd2f737f310a15e984f920730448f99a895893bed351df

src/bitcoin-wallet.cpp

@@ -57,7 +57,7 @@ static bool WalletAppInit(int argc, char* argv[])
     // check for printtoconsole, allow -debug
     LogInstance().m_print_to_console = gArgs.GetBoolArg("-printtoconsole", gArgs.GetBoolArg("-debug", false));
 
-    if (!fs::is_directory(GetDataDir(false))) {
+    if (!CheckDataDirOption()) {
         tfm::format(std::cerr, "Error: Specified data directory \"%s\" does not exist.\n", gArgs.GetArg("-datadir", "").c_str());
         return false;
     }