MFV r354382,r354385: 10601 10757 Pool allocation classes

illumos/illumos-gate@663207adb1669640c01c5ec6949ce78fd806efae
https://github.com/illumos/illumos-gate/commit/663207adb1669640c01c5ec6949ce78fd806efae

10601 Pool allocation classes
https://www.illumos.org/issues/10601

illumos port of ZoL Pool allocation classes. Includes at least these two
commits:
441709695 Pool allocation classes misplacing small file blocks
cc99f275a Pool allocation classes

10757 Add -gLp to zpool subcommands for alt vdev names
https://www.illumos.org/issues/10757

Port from ZoL of
d2f3e292d Add -gLp to zpool subcommands for alt vdev names
Note that a subsequent ZoL commit changed -p to -P
a77f29f93 Change full path subcommand flag from -p to -P

Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Portions contributed by: Håkan Johansson <f96hajo@chalmers.se>
Portions contributed by: Richard Yao <ryao@gentoo.org>
Portions contributed by: Chunwei Chen <david.chen@nutanix.com>
Portions contributed by: loli10K <ezomori.nozomu@gmail.com>
Author: Don Brady <don.brady@delphix.com>

11541 allocation_classes feature must be enabled to add log device

illumos/illumos-gate@c1064fd7ce62fe763a4475e9988ffea3b22137de
https://github.com/illumos/illumos-gate/commit/c1064fd7ce62fe763a4475e9988ffea3b22137de

https://www.illumos.org/issues/11541

After the allocation_classes feature was integrated, one can no longer add a
log device to a pool unless that feature is enabled. There is an explicit check
for this, but it is unnecessary in the case of log devices, so we should handle
this better instead of forcing the feature to be enabled.

Author: Jerry Jelinek <jerry.jelinek@joyent.com>

FreeBSD notes.
I faithfully added the new -g, -L, -P flags, but only -g does something:
vdev GUIDs are displayed instead of device names. -L, resolve symlinks,
and -P, display full disk paths, do nothing at the moment.
The use of special vdevs is backward compatible for read-only access, so
root pools should be bootable, but exercise caution.

MFC after: 4 weeks