MINOR: connection: track mux calls to report their allocation context

MINOR: connection: track mux calls to report their allocation context

Most calls to mux ops were instrumented with a CALL_MUX_WITH_RET() or
CALL_MUX_NO_RET() macro in order to make the current thread's context
point to the called mux and be able to track its allocations. Only
a bunch of harmless mux_ctl() and ->subscribe/unsubscribe calls were
left untouched since useless. But destroy/detach/shut/init/snd_buf
and rcv_buf are now tracked.

It will not show allocations performed in IO callback via tasklet
wakeups however.

In order to ease reading of the output, cmp_memprof_ctx() knows about
muxes and sorts based on the .subscribe function address instead of
the mux_ops address so as to keep various callers grouped.