This exposes ZFS user and group quotas via the normal

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.)

Reviewed by:	delphij, mav
Approved by:	mav
Sponsored by:	iXsystems Inc
Differential Revision:	https://reviews.freebsd.org/D15886