MEDIUM: muxes: Use one callback function to shut a mux stream

MEDIUM: muxes: Use one callback function to shut a mux stream

mux-ops .shutr and .shutw callback functions are merged into a unique
functions, called .shut. The shutdown mode is still passed as argument,
muxes are responsible to test it. Concretly, .shut() function of each mux is
now the content of the old .shutw() followed by the content of the old
.shutr().