Add a "zstream decompress" subcommand

It can be used to repair a ZFS file system corrupted by ZFS bug #12762.
Use it like this:

zfs send -c <DS> | \
zstream decompress <OBJECT>,<OFFSET>[,<COMPRESSION_ALGO>] ... | \
zfs recv <DST_DS>

Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Alan Somers <asomers@gmail.com>
Sponsored-by:  Axcient
Workaround for #12762
Closes #13256

cmd/zstream/zstream_dump.c

@@ -59,7 +59,7 @@ FILE *send_stream = 0;
 boolean_t do_byteswap = B_FALSE;
 boolean_t do_cksum = B_TRUE;
 
-static void *
+void *
 safe_malloc(size_t size)
 {
 	void *rv = malloc(size);