Refactor rb_file_expand_path_internal for dir_string corner cases

* win32/file.c (get_user_from_path):  add internal function that retrieves
  username from supplied path (refactored).
* win32/file.c (rb_file_expand_path_internal):  refactor expansion of user
  home to use get_user_from_path and cover dir_string corner cases.
  [ruby-core:53168] [Bug #8034]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39751 b2dd03c8-39d4-4d8f-98ff-823fe69b080e