Prevent Linux system suspend segfault, fixes #220

A stab at fixing https://github.com/kr/beanstalkd/issues/220.

Customizes the socket event loop for BSD and Linux flavors of Unix to allow Linux to make special considerations for the epoll_event struct. Repeats more code than I'd like, but I like this approach better than the preproccessor alternative.

Build and CT tests succeed on Ubuntu 12.04 and OSX Lion. Also ran [beanstalk_integration_tests](https://github.com/gemeraldbeanstalk/beanstalk_integration_tests) against the Ubuntu build and things were as expected there.