MFC r352591: MFZoL: Retire send space estimation via ZFS_IOC_SEND

Add a small wrapper around libzfs_core's 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.

Obtained from: ZoL
Obtained from: zfsonlinux/zfs@cf7684bc8d57
Author: loli10K <ezomori.nozomu@gmail.com>