MINOR: stconn: add a new pair of sf functions {bs,fs}.debug_str

MINOR: stconn: add a new pair of sf functions {bs,fs}.debug_str

These are passed to the underlying mux to retrieve debug information
at the mux level (stream/connection) as a string that's meant to be
added to logs.

The API is quite complex just because we can't pass any info to the
bottom function. So we construct a union and pass the argument as an
int, and expect the callee to fill that with its buffer in return.

Most likely the mux->ctl and ->sctl API should be reworked before
the release to simplify this.

The functions take an optional argument that is a bit mask of the
layers to dump:
  muxs=1
  muxc=2
  xprt=4
  conn=8
  sock=16

The default (0) logs everything available.