Add support for comparing nanoseconds on the receiver.

Add support for comparing nanoseconds on the receiver.

This patch adds the ability to specify --modify-window=-1 (aka -@-1) to
ask rsync to compare files with the full nanosecond timestamps.  The
default is still -@0 for the moment, which ignores nanoseconds in time
comparisons.  Changing the default to -1 would cause a copy from ext4 to
ext3 to constantly compare as different, or a copy there and back again
to do a full copy as it zeroed all the nanosecond times.  Such a change
might be too much of a functional difference for things like backup
solutions to handle without a warning period.  The current plan is to
support nanosecond comparisons for those that want them, and possibly
change the default window value from 0 to -1 at some point in the
future.