Windows support

The web framework itself is pure Carp, but its dependencies are POSIX-only.

**Socket library (bulk of work):**
- `winsock2.h` instead of `sys/socket.h`, `WSAStartup` init
- `closesocket()` instead of `close()`
- `ioctlsocket(FIONBIO)` instead of `fcntl` for non-blocking
- `WSAPoll` instead of kqueue/epoll (IOCP can be a later optimization)
- `TransmitFile()` instead of `sendfile(2)`

**File library (small):**
- `_unlink` instead of `unlink`
- `FindFirstFile`/`FindNextFile` instead of `opendir`/`readdir`

**Time library (trivial):**
- Minor `#ifdef` for `gettimeofday`

Everything else (json, uri, http, orm, log, strbuf, bufio, web) is pure Carp and needs no changes.