MINOR: mux: Add a new method to get informations about a mux.

MINOR: mux: Add a new method to get informations about a mux.

Add a new method, ctl(), to muxes. It uses a "enum mux_ctl_type" to
let it know which information we're asking for, and can output it either
directly by returning the expected value, or by using an optional argument.
"output" argument.
Right now, the only known mux_ctl_type is MUX_STATUS, that will return 0 if
the mux is not ready, or MUX_STATUS_READY if the mux is ready.

We probably want to backport this to 1.9 and 2.0.