Fix upgrade command on some Linux systems

[jbaiter]

Previously the still open executable file was directly written to, which caused the program to abort with a text file busy message on some systems.

Now the following approach is taken:

1. Move the old file to a backup path
2. Try to write to the original path
3. If opening/writing the original path fails: Move backup to old location
4. If upgrade is successful: Remove backup file

[kotp]

It really helps to have the "fixes #288" in the body of the commit as compared to the title or summary as it links there in the web gui. This is more a complaint to Github itself than anyone that puts that in the title though.

[jbaiter]

Sorry for that, I somewhat mindlessly retyped the original commit title.

[kotp]

It's the functionality of that statement, when combined into master branch, it will auto-close the issue, so it is useful. Unsure if it closes from the first line or not, but in the body on the web interface, it links. So a couple of functional things for having it. I think it should link in the title as well, not just in the body, but that may just be me.

[Tonkpils]

Thanks for this PR! I'll take a look at it soon.

[Tonkpils]

I tested this on Linux and OSX and works great