process.c: avoid EINTR from Process.spawn

* process.c (send_child_error): retry write on EINTR to fix
  occasional Errno::EINTR from Process.spawn.
* process.c (recv_child_error): retry read on EINTR to fix
  occasional Errno::EINTR from Process.spawn.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44706 b2dd03c8-39d4-4d8f-98ff-823fe69b080e