process.c: remove cloexec setting

* process.c (rb_fork_internal): remove cloexec setting on pipes
  created by rb_cloexec_pipe.  patch by normalperson (Eric Wong) at
  [ruby-core:56523].  [Bug #8769]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43008 b2dd03c8-39d4-4d8f-98ff-823fe69b080e