Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Modify the NFS clients and the NLM so that the NLM can be used
by both clients. Since the NLM uses various fields of the
nfsmount structure, those fields were extracted and put in a
separate nfs_mountcommon structure stored in sys/nfs/nfs_mountcommon.h.
This structure also has a function pointer for a function that
extracts the required information from the mount point and nfs vnode
for that particular client, for information stored differently by the
clients.

Reviewed by: jhb
MFC after: 2 weeks

Move sys/nfsclient/nfs_lock.c into sys/nfs and build it as a separate
module that can be used by both the regular and experimental nfs
clients. This fixes the problem reported by jh@ where /dev/nfslock
would be registered twice when both nfs clients were used.
I also defined the size of the lm_fh field to be the correct value,
as it should be the maximum size of an NFSv3 file handle.

Reviewed by: jh
MFC after: 2 weeks

For reasons unknown, the nfs locking code used a fifo to send requests to
userland and a dedicated system call to get replies.

The vnode-bypass of fifos broke this into a panic.

Ditch all the magic and create a device /dev/nfslock instead, and
use that for both directions apart from the shorter path, this is
also faster because the device driver runs Giant free using the
vnode bypass.

Noticed by: marcel

Remove a case of exposing 'struct ucred' to userspace. Use a struct xucred
for LOCKD_MSG instead.

Requested by: rwatson

Add IPv6 support.

Submitted by: Jean-Luc Richier <Jean-Luc.Richier@imag.fr>

o Modify nfslockdans() to accept a thread reference instead of a proc
reference: with td->td_ucred, it will be desirable to authorize
based on td->td_ucred, rather than p->p_ucred.
o Since the same variable 'p' was later used with pfind() on the target
process for the wakeup, introduce a new local variable 'targetp'
to use instead.

Obtained from: TrustedBSD Project
Sponsored by: DARPA, NAI Labs

Cleanup and split of nfs client and server code.
This builds on the top of several repo-copies.

Implement client side NFS locks.

Obtained from: BSD/os
Import Ok'd by: mckusick, jkh, motd on builder.freebsd.org