MEDIUM: proxy/spoe: Add a SPOP mode

MEDIUM: proxy/spoe: Add a SPOP mode

The SPOE was significantly lightened. It is now possible to refactor it to
use a dedicated multiplexer. The first step is to add a SPOP mode for
proxies. The corresponding multiplexer mode is also added.

For now, there is no SPOP multiplexer, so it is only declarative. But at the
end, the SPOP multiplexer will be automatically selected for servers inside
a SPOP backend.

The related issue is #2502.