Keep modtime when copying files

[Kai Grossjohann]

The copy-file function has an optional arg KEEP-TIME which, if
enabled, means that the new copy should have the same timestamp
(last-modified time) as the original.

I can think of several ways to do it.  One is to use the "touch"
command on the remote host to set the time, and the other is to try to
invoke rcp/scp/rsync/... with the right args to keep the time.

What do people think?

Using touch has the advantage that it works for inline methods, too.
But the syntax of touch might be different -- Tramp would have to test
for it.

Kai