rel-path, path-equal: native Windows fixes.

The checks for native Windows are incorrect, plus there
are some issues in the path-volume function.

We cannot check for native Windows at macro-expansion time simply by
calling (find #\\ path-sep-chars) because we compile on Cygwin where
that is false. What we must do is check for being on Windows at
macro-expansion time, and then in the "yes" branch of that decision, the
code must perform the path-sep-char test at run-time. In the "no"
branch, we can output smaller code that doesn't deal with Windows.

* stdlib/copy-file.tl (if-windows, if-native-windows): New macro, which
give a clear syntax to the above described testing.
(path-split): Use if-native-windows.
(path-volume): Use if-native-windows. In addition, fix some broken
tests. The tests for a UNC path "//whatever" cannot just test that the
first components are "", because that also matches the path "/".
It has t be that the first two components are "", and there are more
components. A similar issue occurs in the situation when there is
a drive letter. We cannot conclude that if the component after the
drive letter is "", then it's a drive absolute path, because that
situation occurs in a path like "c:" which is relative.
We also destructively manipulate the path to splice out the volume
part and turn it into a simple relative or absolute path. This is
because the path-simplify function deosn't deal with the volume prefix;
its logic like eliminating .. navigations from root do not work if the
prefix component is present.
(rel-path): We handle a missing error case here: one path has volume
prefix and the other doesn't. Also the error cases that can only occur
on Windows are wrapped with if-windows to remove them at compile time.

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