The vinactive() call in vgonel() may start writes for the dirty pages,
creating delayed write buffers belonging to the reclaimed vnode.  Put
the buffer cleanup code after inactivation.

Add asserts that ensure that buffer queues are empty and add BO_DEAD
flag for bufobj to check that no buffers are added after the cleanup.
BO_DEAD is only used by INVARIANTS-enabled kernels.

Reported and tested by:	pho (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Only sleep interruptible while waiting for suspension end when
filesystem specified VFCF_SBDRY flag, i.e. for NFS.

There are two issues with the sleeps.  First, applications may get
unexpected EINTR from the disk i/o syscalls.  Second, interruptible
sleep allows the stop of the process, and since mount point is
referenced while thread sleeps, unmount cannot free mount point
structure' memory, blocking unmount indefinitely.

Even for NFS, it is probably only reasonable to enable PCATCH for intr
mounts, but this information is currently not available at VFS level.

Reported and tested by:	pho (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Add facility to stop all userspace processes.  The supposed use of the
feature is to quisce the system before suspend.

Stop is implemented by reusing the thread_single(9) with the special
mode SINGLE_ALLPROC.  SINGLE_ALLPROC differs from the existing
single-threading modes by allowing (requiring) caller to operate on
other process.  Interruptible sleeps for !TDF_SBDRY threads are
suspended like SIGSTOP does it, instead of aborting the sleep, like
SINGLE_NO_EXIT, to avoid spurious EINTRs on resume.

Provide debugging sysctl debug.stop_all_proc, which causes total stop
and suspends syncer, while waiting for variable reset for resume.  It
is used for debugging; should be removed after the real use of the
interface is added.

In collaboration with:	pho
Discussed with:	avg
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks