FreeBSD has a zfsbootcfg command that wants zpool_nextboot in libzfs.
Add the function to FreeBSD's libzfs_compat.c, and while here move
the prototype for zfs_jail out of param.h in FreeBSD's SPL and into
libzfs.h under an ifdef for FreeBSD, where the prototype for
zpool_nextboot joins it.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes#10524
fb91f0367e