TCP_KEEPINVTL and TCP_KEEPIDLE - Socket Keep Alives not working

[Cary Lewis]

These constants are usually defined in  netinet/tcp.h but are not defined
anywhere in cygwin.

As a result there does not seem to be a way to enable the socket level keep
alives in cygwin.

One result of this is curl's --keepalive-time option does not work, but it
would affect any attempt to enable socket keep alives.

Windows' sockets do support keep alives, and indeed the win32 version of
curl works correctly.

Can someone explain why the TCP keep alive constants are not defined?

Is it possible to add this functionality?

Where in the cygwin source code does the actual posix socket to winsock
translation take place?