Do not use /tmp for default temp folder on Linux

The /tmp directory is tmpfs on Fedora since at least 2012 and perhaps other distros. That means autosave is useless if anything causes a shutdown or reboot, which is more likely than a crash in my experience (unless using a low-end laptop with GPU or cooling issues).

People will say I should have pushed save, but if I am having a hard day and make one mistake in the last 8 hrs of a 48 hr project, well, that is one major use case for autosave, so why not fix it? I am using a large battery backup only around two years old, but it cut off after less than 10 seconds with a small load. Unexpected things like that can happen even with a battery backup which most people do not have. Another case where people may work a long time without saving is when new users, professional or not, evaluate Blender.

Related threads:

• devtalk.blender.org/t/move-autosave-out-of-tmp/10328
• devtalk.blender.org/t/blender-is-not-complying-with-common-standards-amongst-linux-distros-when-it-comes-to-filepaths/17145

The solution is to use a different directory for autosave such as ~/tmp or others suggested there by default.