Improve how we move files in gerbil codebase.

[teggotic]

Create new move-file utility function. It would try calling rename-file, and if it fails with Invalid cross-device link error, i.e. EXDEV syscall errno, it should call a copy-file function instead.
As part of this issue, we should also update gerbil codebase to use new move-file function to prevent errors in some edgecases.

This is the result of the investigation in issue #1277.
Short summary:
While looking at gxtags implementation of the vi format, I found that rename-file call fails with "Invalid cross-device link" error on my system, because I have /tmp mounted as a different file system. Such cases specifically are not supported by rename syscall, which as far as we can tell is used intenally by the gambit's ##rename-file implementation.

[vyzo]

Implementation at #1281