Changes

osmo-io: Put together message buffers when dequeued from tx queue

Write operations may be incomplete. osmo-io processs will remove
complete message buffers after a write operation from msghdr and put
the msghdr with the remaining buffers back into tx_queue.

If the user requests multiple messages buffers per write operation, the
msghdr of an incomplete write may have less message buffers than the
user requested. The remaining buffers are buffers are taken from next
msghdr in the queue, if exists.

Change-Id: I97c366211dd266fd58ec252890ec017d6d334534