security bug: world read/write permissions for `/keybase` on linux (also, that's a non-standard location)

The `post_install.sh` script gives world read-write-execute permission on a directory (`chmod 777 /keybase`).

**edit** I see that I've just duplicated a comment from @ptman: https://github.com/keybase/client/issues/4739#issuecomment-256976014 (searching for "/keybase" was an overmatch as `/` was ignored). But IMO the security matter needs a bigger (separate) spotlight anyway. I suggest using this ticket for the security problem, and https://github.com/keybase/client/issues/4739 for the file location problem.

Start by inspecting post_install.sh and the installation path that applies chmod 777 to /keybase. Determine the intended permissions and verify that installation no longer grants world read, write, and execute access; issue 4739 provides the separate context about the non-standard location.

Written by the indexing model from the issue text.