util: Add rename fallback for copying across devices

[richiejp]

If a file is being moved across devices then rename() won't work. The
file needs to be copied instead.

/tmp and /home are on separate drives on one of my machines. I can't save the config and load extensions without on that machine without this.

[liquidaty]

Thank you, this is helpful. Would like to make a few further changes-- if you'd like to do so please do, otherwise lmk and I will:

1. change fopen read/write to binary ("rb" and "wb")
2. Is there any easy way to add a test? Am guessing no, since it requires a separate physical device. If that is the case (it's not easy), will add an issue to track that missing test
3. on fail, return errno instead of always returning -1

[richiejp]

Is there any easy way to add a test? Am guessing no, since it requires a separate physical device. If that is the case (it's not easy), will add an issue to track that missing test

I think it should be possible on Linux by mounting a filesystem with FUSE or tmpfs. Apparently Github's runners already have multiple file systems mounted that we have read/write access to. However I also think it could be a bit of time sink.

The rest of the changes I made.

[liquidaty]

Great, thank you. Assuming the file could be binary, any reason to not use fread/fwrite over fgets/fputs?

[richiejp]

No reason, I have changed it.