stitch/placer: refactor mkDest helper.
stitch/placer: refactor mkDest helper.

It now uses more of rio's fs abstractions, but more importantly the detection of the wanted mode is now separated out -- this will be relevant in a moment because some of the other placers also look up that exact same thing early on for other logical switches, so it's better if we accept it as an arg in mkDest.

Signed-off-by: Eric Myhre <2346ad27d7568ba9896f1b7da6b5991251debdf2@exultant.us>

polydawn/rio,polydawn/rio