zstream: consolidate shared code

zstream currently contains three identical copies of dump_record(),
which appear to all be drawn from libzfs_sendrecv.c. The original
is marked internal.

This PR adds zstream_util.[hc] and puts the shared code there along with
a couple of other items in common.

No functional changes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Garth Snyder <garth@garthsnyder.com>
Closes #18284

cmd/zstream/zstream_util.h

@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: CDDL-1.0
+/*
+ * CDDL HEADER START
+ *
+ * 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.
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2026 by Garth Snyder. All rights reserved.
+ */
+
+#ifndef	_ZSTREAM_UTIL_H
+#define	_ZSTREAM_UTIL_H
+
+#ifdef	__cplusplus
+extern "C" {
+#endif
+
+#include <stddef.h>
+#include <sys/zfs_ioctl.h>
+#include <sys/zio_checksum.h>
+
+/*
+ * The safe_ versions of the functions below terminate the process if the
+ * operation doesn't succeed instead of returning an error.
+ */
+extern void *
+safe_malloc(size_t size);
+
+extern void *
+safe_calloc(size_t n);
+
+extern int
+sfread(void *buf, size_t size, FILE *fp);
+
+/*
+ * 1) Update checksum with the record header up to drr_checksum.
+ * 2) Update checksum field in the record header.
+ * 3) Update checksum with the checksum field in the record header.
+ * 4) Update checksum with the contents of the payload.
+ * 5) Write header and payload to fd.
+ */
+extern int
+dump_record(dmu_replay_record_t *drr, void *payload, size_t payload_len,
+	zio_cksum_t *zc, int outfd);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  /* _ZSTREAM_UTIL_H */