zfs label bootenv should store data as nvlist

nvlist does allow us to support different data types and systems.

To encapsulate user data to/from nvlist, the libzfsbootenv library is
provided.

Reviewed-by: Arvind Sankar <nivedita@alum.mit.edu>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #10774

include/libzfsbootenv.h

@@ -0,0 +1,41 @@
+/*
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ */
+
+/*
+ * Copyright 2020 Toomas Soome <tsoome@me.com>
+ */
+
+#ifndef _LIBZFSBOOTENV_H
+#define	_LIBZFSBOOTENV_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef enum lzbe_flags {
+	lzbe_add,	/* add data to existing nvlist */
+	lzbe_replace	/* replace current nvlist */
+} lzbe_flags_t;
+
+extern int lzbe_nvlist_get(const char *, const char *, void **);
+extern int lzbe_nvlist_set(const char *, const char *, void *);
+extern void lzbe_nvlist_free(void *);
+extern int lzbe_add_pair(void *, const char *, const char *, void *, size_t);
+extern int lzbe_remove_pair(void *, const char *);
+extern int lzbe_set_boot_device(const char *, lzbe_flags_t, const char *);
+extern int lzbe_get_boot_device(const char *, char **);
+extern int lzbe_bootenv_print(const char *, const char *, FILE *);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _LIBZFSBOOTENV_H */