Kgio 2.7.4 - kinder, gentler I/O for Ruby + Unix

kgio provides non-blocking I/O methods for Ruby without raising
exceptions on EAGAIN and EINPROGRESS. It is intended for use with the
Unicorn and Rainbows! Rack servers, but may be used by other
applications (that run on Unix-like platforms).


Fix build for platforms lacking both TCP_CORK and TCP_NOPUSH
There are many test case fixes and cleanups, too.