libzpool: add BE_POSIX_VENDOR for userspace bootenv

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

include/sys/zfs_bootenv.h

@@ -31,6 +31,7 @@ extern "C" {
 #define	BE_FREEBSD_VENDOR	"freebsd"
 #define	BE_GRUB_VENDOR		"grub"
 #define	BE_LINUX_VENDOR		"linux"
+#define	BE_POSIX_VENDOR		"posix"
 
 #include <sys/zfs_bootenv_os.h>