Rename hole_birth tunable to match OpenZFS

OpenZFS decided that ignore_hole_birth was too imprecise and
incorrect a name (and went with send_holes_without_birth_time).
Rename it in ZoL too, while keeping the name "ignore_hole_birth"
pointing to the same variable for existing consumers.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #5239