Ruby

deduplicate File::NULL string

"/dev/null" is a common sight for pre-1.9.3-compatible code
targeting *nix systems, so deduplicate it here, as well.

• file.c (Init_File): use fstring for File::NULL

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57996 b2dd03c8-39d4-4d8f-98ff-823fe69b080e