singleton methods.

* object.c (rb_mod_to_s): better description.

* hash.c (env_select): should path the assoc list.

* process.c (rb_syswait): thread kludge; should be fixed to
  support native thread.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@2128 b2dd03c8-39d4-4d8f-98ff-823fe69b080e