USB 3 / eSATA support

[John Levine]

I have an two-disk external box with both USB and eSATA interface.  Go
with eSATA, which is better supported as a disk.  I use mine as a ZFS
mirror.

I have a SiI3124 SATA controller which isn't recognized by the generic kernel, but works
fine once I put a suitable hint in loader.conf:

# for external SATA
siis_load="YES"

I also have three USB disks configured as a ZFS RAID which I use for
backups.  It works OK, but I wouldn't want to depend on it from day to
day.

R's,
John