unix package patch to add support for setting effective uid/gid

Hello, I believe that it would be useful for the unix package to support setting the effective user-id and group-id of a process. By setting the effective id instead of the real id you can toggle between real and set-user-id. It seems like a simple oversight that these functions have been left out of the unix package. I have attached the darcs patch for these function. This patch does not explicitly bump the version number as I thought it might be integrated with other patches before a new release (That could be wrong). -- Eric Mertens Galois, Inc.