Retire send space estimation via ZFS_IOC_SEND

Retire send space estimation via ZFS_IOC_SEND

Add a small wrapper around libzfs_core`lzc_send_space() to libzfs so
that every legacy ZFS_IOC_SEND consumer, along with their userland
counterpart estimate_ioctl(), can leverage ZFS_IOC_SEND_SPACE to
request send space estimation.

The legacy functionality in zfs_ioc_send() is left untouched for
compatibility purposes.

Reviewed by: Thomas Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6029