RFD: Rework/extending functionality of mdev

[Cathey, Jim]

>Stream-writes [pipe] are not atomic, and your message can theoretically get
>cut in half and interleaved with another process writing the same fifo.

Any pipe, whether named or not, IS atomic so long as the datagrams
in question are smaller than PIPE_BUF in size.  This has been true
since Day 1, in every Unix worthy of the name.  You have to be
careful on the reads, though: you need to embed the size of the
datagram into itself so that you can be sure you don't get them
packed together.  If the datagrams are of fixed size, then
you don't even need this.

Most of the pipes I use this way have a datagram whose first
field is the size.  Atomic write(2) of each datagram into
the pipe.  The reader does a read(2) of the size field, followed
by a read sized to get the rest of the datagram.  No muss,
no fuss, and pretty darned fast, too.  And it works _everywhere_,
on every Unix/Linux/whateverix version known to Man.

-- Jim