[libos] Support TCP level socket options

[iyzhang]

Context

Currently, we only support SO_* options on sockets. We should also support TCP_* options. In particular, the keep alive flag is set at the socket level on both Windows and Linux, but Linux sets the keep alive parameters at the TCP level, while Windows uses a socket level data structure.

Proposed Solution

Unify these into our config files and figure out which direction we prefer (and what the correct hierarchy is).

Alternative Solutions

Give up and go home