head's up: disk physical paths changing

I'm planning to commit a change to the format of disk physical paths,
as reported by DIOCGPHYSPATH, and viewable by "diskinfo -v".  This
will only affect disks in a SES enclosure (which are most SAS
enclosures with more than about 8 slots).

Technically, this is a backwards-incompatible change.  But I only know
of two cases that are likely to be affected:
* If anybody uses a /dev/enc@... device alias in /etc/fstab, they'll
need to update it.
* If anybody is using zfsd with autoreplace-by-physical-path, *and*
happens to have a disk missing at the time they install this change,
then zfsd won't autoreplace it when they replace the disk.  They'll
have to replace the disk manually with "zpool replace".

If you think this will be a problem, or if you've thought of any more
cases I haven't, then speak up.  I'll commit in 2 weeks if nobody
objects.

https://reviews.freebsd.org/D39141