MINOR: muxes: Add a flag to notify a mux does not support any upgrade

MX_FL_NO_UPG flag may now be set on a multiplexer to explicitly disable
upgrades from this mux. For now, it is set on the FCGI multiplexer because
it is not supported and there is no upgrade on backend-only multiplexers. It
is also set on the H2 multiplexer because it is clearly not supported.
3 files changed