[dwm] make setlayout toggle

[Neale Pickett]

This simple modification to setlayout causes a binding to toggle if it's
already in the requested layout.

void
setlayout(const Arg *arg)
{
  sellt ^= 1;
  if(arg && arg->v && (arg->v != lt[sellt]))
    lt[sellt] = (Layout *)arg->v;
  if(sel)
    arrange();
  else
    drawbar();
}

Now Alt-M toggles in and out of Monocle, etc.  I far prefer this
behavior, YMMV.

Neale