process.c: use UTF-8

* process.c (open): use UTF-8 version function to support
  non-ascii path properly.  [ruby-core:63185] [Bug #9946]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46441 b2dd03c8-39d4-4d8f-98ff-823fe69b080e