Fallback to TMPDIR if XDG_RUNTIME_DIR is not set

[triarius]

On most macOS setups, XDG_RUNTIME_DIR won't exist, but rather than creating something in the home directory, we can fall back to TMPDIR. Then, users won't have to find ways to set XDG_RUNTIME_DIR manually if they want to keep their home directory clean. Setting it manually can have some undesirable consequences, as outlined in the issue.

This approach is intended to keep the existing behaviour for users who have XDG_RUNTIME_DIR set. If they don't, and have TMPDIR set, then their runtimeDir will change. Otherwise, it will do nothing for them.

There may be users with neither XDG_RUNTIME_DIR nor TMPDIR set. I think we will be happy for them to make another PR if they don't want the runtimeDir in their home directory.

Fixes: #833

[fsouza]

Thanks for contributing!