CLEANUP: muxes: do not use a dynamic trash in list_mux_protos()

CLEANUP: muxes: do not use a dynamic trash in list_mux_protos()

Let's not use a trash there anymore. The function is called at very
early boot (for "haproxy -vv"), and the need for a trash prevents the
arguments from being parsed earlier. Moreover, the function only uses
a FILE* on output with fprintf(), so there's not even any benefit in
using chunk_printf() on an intermediary variable, emitting the output
directly is both clearer and safer.