Implement "soft close"

Relax file consistency semantics to avoid
writing cached data (sync) on file close.

Add a compile-time option to avoid a sync on
file close.

A file close will not necessarily write all data
to the file. PIO can rearrange and update
the file asynchronously.

This would allow PIO to buffer more data and
possibly improve I/O performance for certain
file I/O patterns.