Port remotesrv and HTTP remote client to Windows (Winsock + Win32 threads)

[timsehn]

Summary

PR #200 added Windows build support but stubbed out the networking code:

• doltlite_http_remote.c: doltliteRegisterHttpRemote() is a no-op on Windows
• doltlite_remotesrv.c: doltliteServerCreate() returns NULL with error on Windows

This means dolt_remote_add / push / fetch / clone and the standalone doltlite-remotesrv binary don't work on Windows.

What needs to happen

1. Replace BSD socket calls with Winsock2 (WSAStartup, socket, connect, send, recv, closesocket)
2. Replace pthread_create/pthread_join with CreateThread/WaitForSingleObject
3. Replace poll() with WSAPoll() (Vista+) or select()
4. Add #ifdef _WIN32 / #else branches or use a thin abstraction layer
5. Link with -lws2_32 on Windows

Files

• src/doltlite_http_remote.c — HTTP client (sockets only, no threads)
• src/doltlite_remotesrv.c — HTTP server (sockets + threads + poll)