various clean ups for midi and sequencer:

midi specific:
- add reference counting for midi operations, and ensure that
  detach waits for other threads to complete before tearing
  down the device completely.
- in detach, halt midi callouts before destroying them
- re-check sc->dying after sleeping in midiread()
- in real_writebytes(), make sure we're open and not dying
- make sure we drop the interrupt lock before calling any code
  that may want to check thread locks.  this is now safe due to
  the above changes.

sequencer specific:
- avoid caching the midi softc in the sequencer softc.  instead,
  every time we want to use it, look it up again and make sure
  it still exists.

this fixes various crashes i've seen in the usb midi code when
detaching the umidi while it is active.
Do not warn about relative path because of trailing slash

libpuffs calls realpath() to obtain an absolute path to use for mounting.
If the obtained path is different from the one given by the caller, a
warning is issued. This included the situation where the path passed by
the caller just have trailing slashes, a situation where we just want them
to be striped without a warning.