Deduplicate bitcoind and bitcoin-qt init code

Add common InitConfig function to deduplicate bitcoind and bitcoin-qt code
reading config files and creating the datadir.

There are a few minor changes in behavior:

- In bitcoin-qt, when there is a problem reading the configuration file, the
  GUI error text has changed from "Error: Cannot parse configuration file:" to
  "Error reading configuration file:" to be consistent with bitcoind.
- In bitcoind, when there is a problem reading the settings.json file, the
  error text has changed from "Failed loading settings file" to "Settings
  file could not be read" to be consistent with bitcoin-qt.
- In bitcoind, when there is a problem writing the settings.json file, the
  error text has changed from "Failed saving settings file" to "Settings file
  could not be written" to be consistent with bitcoin-qt.
- In bitcoin-qt, if there datadir is not accessible (e.g. no permission to read),
  there is an normal error dialog showing "Error: filesystem error: status:
  Permission denied [.../settings.json]", instead of an uncaught exception

ci/test/06_script_b.sh

@@ -42,6 +42,7 @@ if [ "${RUN_TIDY}" = "true" ]; then
   ( CI_EXEC run-clang-tidy -quiet "${MAKEJOBS}" ) | grep -C5 "error"
   export P_CI_DIR="${BASE_BUILD_DIR}/bitcoin-$HOST/"
   CI_EXEC "python3 ${DIR_IWYU}/include-what-you-use/iwyu_tool.py"\
+          " src/common/init.cpp"\
           " src/common/url.cpp"\
           " src/compat"\
           " src/dbwrapper.cpp"\