freebsd: add ifdefs around legacy ioctl support

Require that ZFS_LEGACY_SUPPORT be defined for legacy ioctl support to
be built. For now, define it in zfs_ioctl_compat.h so support is always
built. This will allow systems that need never support pre-openzfs
tools a mechanism to remove support at build time. This code should
be removed once the need for tool compatability is gone.

No functional change at this time.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #14127