process.c: vfork may not work with pthread

* process.c (retry_fork_async_signal_safe): vfork may not work
  with pthread always even if fork works with it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47439 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-09-07

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47440 b2dd03c8-39d4-4d8f-98ff-823fe69b080e