- Use a secure path for sockets (some code used under license from
tmux).
- Provide the secure path in the environment as $__fish_runtime_dir.
- Link the new path to the old path to ease migration from earlier
versions.
Closes#1359.
After installing fish built from or after this commit, you MUST
terminate all running fishd processes (`killall fishd`, `pkill fishd`
or similar). Distributors are encouraged to do this from within their
packaging scripts. fishd will restart automatically, and no data should
be lost.