POSIX message queues allow local processes to exchange data in the form
of messages. This API is distinct from that provided by System V
message queues, but provides similar functionality.
Fix POSIX_MQ#notify(&block) usage, this regression was
introduced in 0.4.0 and our tests for it were broken, as well.
Not many people use POSIX_MQ.notify(&block), however, as it’s only
supported under Linux at the moment and “POSIX_MQ.notify = signal” is
more portable and efficient. Of course, it’s possible to use POSIX
message queues without notifications at all in blocking mode. (and Linux
and FreeBSD also allow POSIX_MQ objects to be IO.select-ed).