Currently lzc_receive() requires that its snapname argument is a snapshot name
(contains '').
@zfs receive allows to specify just a dataset name and would try to deduce the
snapshot name from the stream.
I propose to allow lzc_receive() to do the same.
That seems to be quite easy to implement, it requires only a small amount of
logic, it does not require any additional system calls or any additional data
from the stream.
The benefit is that the new behavior would allow to keep the snapshot names the
same between the sender and receiver at zero cost, without a need to pass the
names out of band.