libzfs: Make zfs_cmd_t initialization consistent, use zfs_ioctl

The clang version 8.0.1 shipped in FreeBSD 12.1-RELEASE also oddly
throws a warning that is treated as an error on the initialization of
the zc struct in zpool_nextboot.

The zpool_nextboot code from FreeBSD was not updated to use zfs_ioctl.

Switch ioctl to zfs_ioctl in and use {"\0"} to initialize the struct.
Do a consistency pass for zfs_cmd_t initialization.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10539