Must ensure $multirust_dir isn't world-writable in order to avoid symlink race attack

[nodakai]

Cf. http://www.linuxsecurity.com/content/view/115462/151/

1. Add -m 755 to mkdir -p $multirust_dir or wrap it with a subshell with umask 022.
2. If the existing $multirust_dir should be world-writable, multirust should abort normal operation and ask the user to take some actions.

[brson]

Yes, good suggestion.