MFC r336017,r338799

MFC r336017,r338799

r336017
	This exposes ZFS user and group quotas via the normal
	quatactl(2) mechanism.  (Read-only at this point, however.)
	In particular, this is to allow rpc.rquotad query quotas
	for NFS mounts, allowing users to see their quotas on the
	hosts using the datasets.

	The changes specifically:

	* Add new RPC entry points for querying quotas.
	* Changes the library routines to allow non-UFS quotas.
	* Changes rquotad to check for quotas on mounted filesystems,
	rather than being limited to entries in /etc/fstab
	* Lastly, adds a VFS entry-point for ZFS to query quotas.

	Note that this makes one unavoidable behavioural change: if quotas
	are enabled, then they can be queried, as opposed to the current
	method of checking for quotas being specified in fstab.  (With
	ZFS, if there are user or group quotas, they're used, always.)

r338799
	Author: kib

	Fix ZFS VFS op quotactl to follow busy protocol.

Approved by: mav
Sponsored by: iXsystems, inc