fusefs: WIP fixing writeback cacheing

fusefs: WIP fixing writeback cacheing

The current "writeback" cache mode, selected by the
vfs.fusefs.data_cache_mode sysctl, doesn't do writeback cacheing at all.  It
merely goes through the motions of using buf(9), but then writes every
buffer synchronously.  This commit:

* Enables delayed writes when the sysctl is set to writeback cacheing
* Fixes a cache-coherency problem when extending a file whose last page has
  just been written.
* Removes the "sync" mount option, which had been set unconditionally.
* Adjusts some SDT probes
* Adds several new tests that mimic what fsx does but with more control and
  without a real file system.  As I discover failures with fsx, I add
  regression tests to this file.
* Adds a test that ensures we can append to a file without reading any data
  from it.

This change is still incomplete.  Clustered writing is not yet supported,
and there are frequent "panic: vm_fault_hold: fault on nofault entry" panics
that I need to fix.

Sponsored by:	The FreeBSD Foundation