Don't overrun req_array when advancing.
Don't send request with more than 1m of data; split
iovecs as necessary to accomplish this.

The current version of accelio doesn't appear to notify us
on the extra requests but only gives one "message delivered"
for the whole chain.  Needs more investigation (are we
being notified on the first instead of the last message in
the chain?), but for now,
	Don't claim extra references on xmsg for extra req's.
	Don't set "user_context" in extra req's.

Signed-off-by: Marcus Watts <mdw@cohortfs.com>
Complain then discard packets that we can't possibly process.

In case of error, don't release references we don't take on xmsg.
Accelio is only giving us one notification for a chain of messages,
so there's no reason to take more than one reference on the message.
(But see immediate previous commit to src/msg/XioMessenger.cc for more on this.)

Signed-off-by: Marcus Watts <mdw@cohortfs.com>