unicorn 3.5.0 - very minor improvements

A small set of small changes but it's been more than a month
since our last release.  There are minor memory usage and
efficiently improvements (for graceful shutdowns).  MRI 1.8.7
users on *BSD should be sure they're using the latest patchlevel
(or upgrade to 1.9.x) because we no longer workaround their
broken stdio (that's MRI's job :)