Implement GEOM based media device classification. You'll notice a few
different things from this commit:
+ More devices. Devices that were previously ignored are now present.
+ Faster device scanning. "There is no try, only Do" -- f_device_try()
  is no longer the basis of device scanning as GEOM provides [nearly]
  all devices (doesn't provide network devices).
+ More information available as non-root. Usually you have to be root
  to do things like taste filesystems, and that limits the amount of
  information available to non-root users; with GEOM, we see all even
  running unprivileged as the brunt of information (except for so-
  called ``dangerously dedicated'' file systems) is represented by the
  `kern.geom.confxml' sysctl(8) MIB.
NB: Only really useful for external scripts that use the API and run as
    non-root; where this code is used in bsdconfig(8) and bsdinstall(8)
    you are running as root so can detect even ``dangerously dedicated''
    file systems that are not present in GEOM; e.g., no PART class for
    a DOS filesystem written directly to disk without partition table).
+ No more use of legacy tools such as diskinfo(8) to get disk capacity
  or fdisk(8) to see partitions.

MFC after:	1 week
Update zfsboot to coincide with r264840 to bsdconfig(8) adding
GEOM support (thereby adding GEOM support to the disk selection
menu of bsdinstall(8)'s `zfsboot' module updated herein).

MFC after:	1 week
X-MFC-with:	264840
Modify the NFSv4 client's Pathconf RPC (actually a Getattr Op.)
so that it only does the RPC for names that are answered by the RPC.
Doing the RPC for other names is harmless, but unnecessary.

MFC after:	2 weeks