f81233524f changed a use of package 'path' to 'filepath'.
Restore it back to 'path', with a comment.
Also, use the os.Executable-based fallback name in the case where the
binary itself doesn't have Go module information. That was overlooked in
the original code.
We were using the Go 'path' module, which apparently doesn't handle
backslashes correctly. path/filepath does.
However, the main bug turned out to be that we were not calling .Base()
on the path if version.ReadExe() fails, which it seems to do at least
on Windows 7. As a result, our logfile persistence was not working on
Windows, and logids would be regenerated on every restart.
Affects: #620
Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>
