[iobuf] Add iob_split() to split an I/O buffer into portions

[iobuf] Add iob_split() to split an I/O buffer into portions

RNDIS devices may provide multiple packets encapsulated into a single
message.  Provide an API to allow the RNDIS driver to split an I/O
buffer into smaller portions.

The current implementation will always copy the underlying data,
rather than splitting the buffer in situ.

Signed-off-by: Michael Brown <mcb30@ipxe.org>