Link coreutils commands that do not conflict

The coreutils formula installs all of its commands with a prefix of 'g'
in order to prevent collision with OSX commands. This change preserves
this behavior and creates an additional, non-prefixed link to commands
which do not have a conflicting OSX version, e.g., ```dircolors```.

An alternative approach would be to list the ```gnubin``` at the end of
the PATH, but this requires an additional manual installation step.
**Including the extra links eliminates this manual step and means the
missing OSX commands will 'just work' -- presumably the reason the user
installed the formula in the first place.**

Links are also created for the man pages to non-conflicting commands.

The list of non-conflicting commands is built by hand. This was done in
favor of some form of automation so that: 1) it is obvious which
commands are made available through this change, and 2) so that the list
can be easily modified in future releases in case a command is deemed
dangerous or otherwise not a good idea to have readily available.