Fix stderr forwarding

Output from webkit_server can be silenced by initializing a Connection with the stdout option set to nil. This broke in 2ceab4e525f2326d5058ebb268cdfa82329ac132.

This PR also drops forwarding of stdout and changes the stdout option to stderr. Once the server has started, nothing is written to stdout. 2ceab4e525f2326d5058ebb268cdfa82329ac132 caused stderr to be forwarded to stdout, which was incorrect behavior.