[System]: Rename the internal class 'ChunkStream' into 'MonoChunkStream'. (#4803)

[System]: Rename the internal class 'ChunkStream' into 'MonoChunkStream'. (#4803)

* [System]: Fully stub-out SslStream on platforms where it's not supported.

* Remove 'SECURITY_DEP' conditional from LocalCertificateSelectionCallback.

* Add the internal delegates and remove 'SECURITY_DEP' from SslStream.platformnotsupported.cs.

* In SslStream.cs, we now provide the full API when '!SECURITY_DEP' using the
PlatformNotSupportedException()-based implementation from SslStream.platformnotsupported.cs

* [System]: Rename the internal class 'ChunkStream' into 'MonoChunkStream'.

Our 'ChunkStream' is used by 'WebConnection', so it will stay after using
HttpListener from corefx.