kgio 2.7.0 - minor updates

When running under Ruby trunk/2.0.0dev, all IO objects created
by kgio will be close-on-exec by default to match the (future)
2.0.0 behavior.  accept()ed sockets in kgio have always been
close-on-exec by default..

Singleton Kgio.accept_* methods are deprecated as the
kgio_accept/kgio_tryaccept methods all take an additional
flags argument.

There are various, test, documentation, and error message
improvements.