* file.c: newly added a class File::Statfs. (experimental)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45661 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
show what filesystem it is on failure

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45662 b2dd03c8-39d4-4d8f-98ff-823fe69b080e