.NET 5.0 not building

`-D net-ver=50` produces the following error:
```
Pointer.hx:46: characters 8-75 : You need to define 'unsafe' to be able to use unsafe code in hxcs.
```

However, after adding `-D unsafe` it also fails to build, complaining about missing networking classes.
```
"Building Javascript..."
"Building Python..."
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Host.hx:26: characters 8-25 : Type not found : cs.system.net.Dns
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Host.hx:28: characters 8-33 : Type not found : cs.system.net.IPHostEntry
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Socket.hx:29: characters 8-43 : Type not found : cs.system.net.sockets.NetworkStream
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Socket.hx:30: characters 8-42 : Type not found : cs.system.net.sockets.ProtocolType
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Socket.hx:31: characters 8-41 : Type not found : cs.system.net.sockets.SocketFlags
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Socket.hx:32: characters 8-44 : Type not found : cs.system.net.sockets.SocketShutdown
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Socket.hx:33: characters 8-40 : Type not found : cs.system.net.sockets.SocketType
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Socket.hx:35: characters 8-36 : Type not found : cs.system.net.sockets.Socket
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Socket.hx:44: characters 19-31 : Type not found : NativeSocket
src/operators/FetchFileOperator.hx:17: characters 57-67 : String should be Int
src/operators/FetchFileOperator.hx:17: characters 93-103 : String should be Int
C:\HaxeToolkit\haxe\std/cs/_std/sys/net/Host.hx:35: characters 38-49 : Type not found : IPHostEntry
```

Reproduce the build with `-D net-ver=50`, then with `-D unsafe`, and review the networking errors in the hxcs standard-library files named in the output. Inspect `src/operators/FetchFileOperator.hx` for the reported String/Int errors, then verify that the .NET 5 target builds successfully.

Written by the indexing model from the issue text.