Don't erase final byte of envblock

When we copy the envblock's contents out, we currently treat it as 
a normal C string. However, this functionality is supposed to more
closely emulate interacting with a file. As a consequence, we were 
incorrectly truncating the contents of the envblock by replacing 
the final byte of the buffer with a null character.

Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes openzfs#10405

lib/libzfs/libzfs_pool.c

@@ -4539,7 +4539,7 @@ zpool_get_bootenv(zpool_handle_t *zhp, char *outbuf, size_t size, off_t offset)
 		return (0);
 	}
 
-	strlcpy(outbuf, envmap + offset, size);
+	strncpy(outbuf, envmap + offset, size);
 	int bytes = MIN(strlen(envmap + offset), size);
 	fnvlist_free(nvl);
 	return (bytes);