Optionally allow one web client to support multiple browsers

[Pomax]

• allow more than one browser to connect (default: no)
• kill active ws and connect new browser (default: yes) when a new browser tries to connect
• require authentication before accepting connection (default: no)

This should be a refactor first, where we switch over the .browser property in clients to a .browsers array that contains either no, or a single, socket connection.