Remove runfile before writing on Unix platforms to avoid permissions issues

[ethanbb]

I have a bunch of changes in my development fork/branch of this repository, and I'm trying to sort through them now, identify fixes that haven't been included in merged or proposed features yet, and isolate them into feature branches. This is a simple one to start with.

I had an issue at one time with the runfile existing but not being writable (probably since we're using a NAS on a multi-OS domain with some uid issues). Removing the file first shouldn't change the end result (except file metadata) but solved this particular permissions issue.