Prevent ZFS leaking pool free space

When processing async destroys ZFS would leak space every txg timeout
(5 seconds by default), if no writes occurred, until the pool is totally
full. At this point it would be unfixable without a pool recreation.

In addition if the machine was rebooted with the pool in this situation
would fail to import on boot, hanging indefinitely, as the import process
requires the ability to write data to the pool. Any attempts to query the
pool status during the hung import would not return as the import holds
the pool lock.

The only way to import such a pool would be to specify -o readonly=on
to the zpool import.

zdb -bb <pool> can be used to check for "deferred free" size which is where
this lost space will be counted.

MFC after:	3 days
Sponsored by:	Multiplay
Add two sysctl(8) to enable/disable NFSv4 server to check when setting
user nobody and/or setting group nogroup as owner of a file or directory.
Usually at the client side, if there is an username that is not in the
client's passwd database, some clients will send 'nobody@<your.dns.domain>'
in the wire and the NFSv4 server will treat it as an ERROR.
However, if you have a valid user nobody in your passwd database,
the NFSv4 server will treat it as a NFSERR_BADOWNER as its believes the
client doesn't has the username mapped.

Submitted by:	Loic Blot <loic.blot@unix-experience.fr>
Reviewed by:	rmacklem
Approved by:	rmacklem
MFC after:	2 weeks