* Support XDG_CONFIG_HOME for gemrc.

* Support XDG_DATA_HOME for .gem

* Added test for XDG_DATA_HOME

* Do not reuse environmental variable.

* Unify .rdoc path to RDoc.home.

* Support XDG_DATA_HOME for .rdoc

* Ignore exists?

* Extracted config_home path

* Use XDG_CONFIG_HOME for default credential path

* Fixed inconsistency location.

* Fixed the broken tests.

* Support XDG_CONFIG_HOME for irbrc

* Introduce Gem.cache_home as XDG_CACHE_HOME

* Use Gem.cache_home instead of Gem.config_home for the credential file of RubyGems.

* Initialized the old configurations

* Fixed test failure related the configuration initialization

* restore XDG_DATA_HOME

* Fixed the broken examples of bundler with XDG_*

* Do not modify environmental variable on test file

* Use XDG_DATA_HOME insted of XDG_CACHE_HOME for credential file

* stub out Gem.data_home

* Move dir accessor to defaults.rb file

* Use XDG_DATA_HOME for signed gem features

* Use XDG_DATA_HOME for spec cache

* Do not rely on Gem.user_home

* Gem.user_home is always exists. Don't need to use FileUitls.mkdir_p

* Bump support version to RubyGems 3.2.0+

* Removed the needless fallback configuration

* Fixed the inconsistency methods that are find_config_file and config_file

* Use Gem.configuration.credentials_path instead of hard-coded path

* gem_path is always provided

* Removed the duplicated code of find_home

* Also removed the duplicated code of user_home

* use Gem::UNTAINT instead of untaint for surpressing the warnings

* Use File.directory

* Restore XDG_DATA_HOME

* Use File.write

* `RbConfig::CONFIG['ridir']` has been set since 1.9.2

* `ENV["HOME"]` has been defaulted to `USERPROFILE` since 1.9.0,
  and backported to 1.8.5 or later.

https://github.com/ruby/rdoc/commit/a3a614e676

This reverts commit ab7a6e1a1651d82d327d155b78a8e3af1d976707.

The 559dca509d2a98584b09c7d9a6d74749ce793ad7 contains an excess range in
using "require".

Accidentally merged when 89271d4a3733bc5e70e9c56b4bd12f277e699c42
"Adjusted indents".

  It version applied `frozen_string_literal: true`

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60920 b2dd03c8-39d4-4d8f-98ff-823fe69b080e