Cygwin: path handling: never substitute virtual drive with target path

Cygwin: path handling: never substitute virtual drive with target path

Move the drive substitution code after the call to
GetFinalPathNameByHandleW into a local function revert_virtual_drive
and add code to handle non-remote virtual drives, i. e., those
created with the subst command.  (Try to) make sure that virtual
drives are never treated like symlinks.

Fixes: 19d59ce75d53 ("Cygwin: path_conv: Rework handling native symlinks as inner path components")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>