Revert "Check effective credentials of socket peers"

This reverts commit 8412c867a501e3a68e55fef6215e86d3ac9f617b.

Just checking the credentials of the peer turns out to be insufficient.
See https://github.com/fish-shell/fish-shell/issues/1436.
Fix for CVE-2014-2905 - fishd restart required.

 - 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.