More AHCI: port multipliers

Matthew Dillon has initial support in for port multipliers, along with other AHCI work.  It’s not ready for production yet, and he lists the various issues going on, including a need for a different way to mount disks – AHCI changes devicenames from ‘ad’ to ‘da’, which can be a hassle.

Update: hot-swap support, too.

Update update: parallel scans for speed.