libzpool/abd_os: iovec-based scatter abd

This is intended to be a simple userspace scatter abd based on struct
iovec. It's not very sophisticated as-is, but sets a base for something
much more interesting.

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