midi(4): *sleep(9) -> *sleep_nsec(9)

These are straightforward except for the tsleep(9) conversion.  ratchov@
had a bit to say about that code:

https://marc.info/?l=openbsd-tech&m=157665936017373&w=2

> The problem is that close(2) may reset the transmitter before the few
> bytes of its internal buffer is sent on the wire; there's no "wait for
> completion" feature in such simple hardware, so we just wait few
> milliseconds.
>
> The transmitter buffer size is around 16 bytes, the byte rate is 3125
> bytes/second.  So if we wait at least 16B / 3125B/s = 5.12ms, we're
> safe. Waiting 10ms-20ms is enough and is unnoticeable.

Hence, in this diff we wait a flat 20ms in that situation.

ok ratchov@

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