Regardless of future features, this needs to work with
kqueue descriptors across platforms.

Today this will be useful for 3rd-party libraries using
kqueue.  In the future, Ruby may use kqueue natively
and we shall ensure we can wait on it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63421 b2dd03c8-39d4-4d8f-98ff-823fe69b080e