#2 Support for native filecopy and rename

[Colin McCormack]

A vfs is able to provoke expensive fallback simulations
of these commands by either leaving the respective
FileSystem proc pointers NULL, or by returning -1 as
the result of invocation of the commands.

I suggest we add to the tclvfs interface to enable
these facilities, defaulting them to return -1, thus
falling back to the current behavior, but permitting
tclvfs implementations to support native copy and rename.

For some tclvfs implementations, notably mk4vfs, the
native implementations would be far more efficient than
the current fallback.

I will prepare a sample implementation, if this is
acceptable.

[Colin McCormack]

Patch implementing copy and rename