implement TCP Fast Open support (client + server)
Server support just requires exposing one constant for
setsockopt: Kgio::TCP_FASTOPEN
Client support implements a new Kgio::Socket#fastopen method.
This new method wraps the the sendto() syscall. With TCP Fast
Open, the sendto() syscall is overloaded for stream sockets to
implement the functionality of both connect() + write()
Since it only makes sense to use _blocking_ I/O for sendto(),
TFO clients are only supported in Ruby implementations with
native threads.