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

lib/libzpool/include/Makefile.am

@@ -3,5 +3,6 @@ libzpool_sys_HEADERS = \
 	%D%/sys/abd_os.h \
 	%D%/sys/abd_impl_os.h \
 	%D%/sys/trace_zfs.h \
+	%D%/sys/zfs_bootenv_os.h \
 	%D%/sys/zfs_context_os.h \
 	%D%/sys/zfs_debug_os.h

lib/libzpool/include/sys/zfs_bootenv_os.h

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: CDDL-1.0
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or https://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2025, Rob Norris <robn@despairlabs.com>
+ */
+
+#ifndef _ZFS_BOOTENV_OS_H
+#define	_ZFS_BOOTENV_OS_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define	BOOTENV_OS		BE_POSIX_VENDOR
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _ZFS_BOOTENV_OS_H */