file.c: split rb_home_dir

* dir.c (dir_s_home): use rb_home_dir_of and rb_default_home_dir.
* file.c (rb_home_dir_of): split from rb_home_dir() for the home
  directry of the given user, and the user name is a VALUE, not a bare
  pointer.  should raise if the user does not exist.
* file.c (rb_default_home_dir): split from rb_home_dir() for the home
  directry of the current user.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42160 b2dd03c8-39d4-4d8f-98ff-823fe69b080e