Remove the vfs page replacement optimization and its ENABLE_VFS_IOOPT option.

Remove the vfs page replacement optimization and its ENABLE_VFS_IOOPT option.
This never worked properly... that is, the semantics are broken compared to
a normal read or write in that the read 'buffer' will be modified out from
under the caller if the underlying file is.

What is really needed here is a copy-on-write feature that works in both
directions, similar to how a shared buffer is copied after a fork() if either
the parent or child modify it.  The optimization will eventually rewritten
with that in mind but not right now.