cf-socket: enable Win10 `TCP_KEEP*` options with old SDKs

cf-socket: enable Win10 `TCP_KEEP*` options with old SDKs

Define `TCP_KEEP*` macros if they are missing in Windows builds.
To allow using these runtime `setsockopt()` options regardless of
build-time SDK version, when running on Windows 10.0.16299+.

In practice in enables them for builds using mingw-w64 <12, and
MSVC with Windows SDK <10.

Before this patch these runtime options required building curl with
a recent toolchain.

Follow-up to f0de14168a4d1c3a4ed43a04af92c5755c84b9fc #19559

Closes #19999