Enable (unlimited) core dumps when running i3 development versions

Enable (unlimited) core dumps when running i3 development versions

Also prints out useful stuff:

    CORE DUMPS: You are running a development version of i3, so coredumps were
    automatically enabled (ulimit -c unlimited).
    CORE DUMPS: Your current working directory is "/home/michael/i3".
    CORE DUMPS: Your core_pattern is: /tmp/%e.core.%p
    i3 (tree) version 4.0.2-479-g26ab2ac (2011-11-08, branch "next") starting

This does not affect child processes of i3.

The intention of this change is to make debugging easier – it’s one less thing
users of the development version have to worry about when trying to help with
debugging.