[Ecls-list] Nowait semaphore/mailbox diff review

[Matthew Mondor]

There are situations where it's useful to attempt to post a message
without blocking (where data loss is acceptable as for UDP), as well as
cases where it's useful to query an incomming port without blocking if
it's empty.

This diff implements:

MP:TRY-GET-SEMAPHORE: returns NIL if the semaphore could not be
obtained, or the count if it could

MP:MAILBOX-TRY-READ: returns a message if it's available, NIL if the
queue is empty

MP:MAILBOX-TRY-SEND: returns the sent message if it could be queued, or
NIL (mailbox is full)

I'm slightly worried about potential race conditions, this probably has
to be carefully reviewed.

Thanks,
-- 
Matt
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