This is mostly a placeholder; it's not actually clear if a boot
environment makes any sense for userspace. Still, "posix" is the likely
future name of libzpool as a port, and this define is mandatory, so lets
roll with it for now.
Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes#17861
