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_redup.c

@@ -65,7 +65,7 @@ highbit64(uint64_t i)
 	return (NBBY * sizeof (uint64_t) - __builtin_clzll(i));
 }
 
-static void *
+void *
 safe_calloc(size_t n)
 {
 	void *rv = calloc(1, n);
@@ -81,7 +81,7 @@ safe_calloc(size_t n)
 /*
  * Safe version of fread(), exits on error.
  */
-static int
+int
 sfread(void *buf, size_t size, FILE *fp)
 {
 	int rv = fread(buf, size, 1, fp);