net/mosquitto: Fix websockets options

Rename WEBSOCKET to WEBSOCKETS to match the cmake options. Add
WEBSOCKETS_BUILTIN option. I'd like to have WEBSOCKETS control if
websockets is supported at all and WEBSOCKETS_BUILTIN switch between
the bundled and ports versions but the latter doesn't build so mark
BROKEN and open an upstream issue:

https://github.com/eclipse-mosquitto/mosquitto/issues/3478

One effect of these changes is that libwebsockets is no longer a
LIB_DEPENDS for the port; this will become a possibility once
WEBSOCKETS_BUILTIN OFF works again.

PR: 293005
Reported by: alexander@wittig.name