Make ihandle sync behavior runtime-configurable

The actual behavior of FDH_SYNC has changed a bit over the years, and
some people want one behavior, and some want another. Make it possible
to make this choice at runtime with the new -sync option, instead of
making this decision by running with different patches.

Note that FDH_SYNC is not a macro anymore, nor is it an inline
function. While it could be a macro, it would look a bit complex, and
there are some oddities with trying to use vol_io_params inside the
FDH_SYNC expansion (vol_io_params is not declared for LWP, for
example). And having it be an inline function causes problems with
some odd linking dependencies. For example, vlib.a contains volume.o,
but does not contain a definition for DFlushVolume (dir/buffer.c),
which is referenced in volume.o.  'vos' uses vlib.a, but does not
bring in anything that defines DFlushVolume. Currently this appears to
not cause a problem because 'vos' uses nothing from volume.o, so the
dependencies of volume.o don't matter. Adding an inline FDH_SYNC for
platforms that don't support 'static inline' would add a dependency to
volume.o (via vol_io_params), which causes an error for the lack of a
DFlushVolume.

Those are possibly just some problems, and may not be all. So instead,
make it so we don't have to deal with that and just have a normal
function. While FDH_SYNC may be called in a performance-critical
section, the overhead of a real function call is nowhere near the
delay of an actual fsync(), so presumably any overhead doesn't matter.

Change-Id: I23620bd8ac31b9019e9d55cb46ec9f3a75f5675c
Reviewed-on: http://gerrit.openafs.org/9694
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>