Revert "Check effective credentials of socket peers"

This reverts commit aea9ad4965d24ef9c4e346f906194820bac70cc9.

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.
Further fixes to universal variable server socket management

- Change fishd_path to std::string
- Warn, rather than exiting with an error, if the universal variable
  server path is not available, and provide more useful advice.
- Export the new __fishd_runtime_dir variable.
Authenticate connections to web_config service

 - Require all requests to use a session path.
 - Use a redirect file to avoid exposing the URL on the command line, as
   it contains the session path.

Fix for CVE-2014-2914.
Closes #1438.