value strings: pretty printers for flags and enums

This adds zfs_valstr, a collection of pretty printers for bitfields and
enums. These are useful in debugging, logging and other display contexts
where raw values are difficult for the untrained (or even trained!) eye
to decipher.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>

include/sys/zio.h

@@ -167,6 +167,9 @@ typedef enum zio_suspend_reason {
  * This was originally an enum type. However, those are 32-bit and there is no
  * way to make a 64-bit enum type. Since we ran out of bits for flags, we were
  * forced to upgrade it to a uint64_t.
+ *
+ * NOTE: PLEASE UPDATE THE BITFIELD STRINGS IN zfs_valstr.c IF YOU ADD ANOTHER
+ * FLAG.
  */
 typedef uint64_t zio_flag_t;
 	/*