Overwrite existing symlinks with --overwrite

I had some problems and ended up reinstalling homebrew but some symlinks were left in my  `/usr/local`, so I was getting this:

    $ brew link --overwrite suite-sparse
    Linking /usr/local/Cellar/suite-sparse/4.2.0... Warning: Could not link suite-sparse. Unlinking...

    Error: Could not symlink file: /usr/local/Cellar/suite-sparse/4.2.0/include/umfpack_solve.h
    /usr/local/include/umfpack_solve.h may already exist.
    /usr/local/include may not be writable.

`brew link` has `--overwrite` (you can see I tried it above), but that only works when the existing things are files. This small patch makes `--overwrite` work with existing symlinks, too, so you get a nicer error message:

    $ brew link suite-sparse
    Linking /usr/local/Cellar/suite-sparse/4.2.0... Warning: Could not link suite-sparse. Unlinking...

    Error: Could not symlink file: /usr/local/Cellar/suite-sparse/4.2.0/include/umfpack_solve.h
    Target /usr/local/include/umfpack_solve.h already exists. You may need to delete it.
    To force the link and delete this file, do:
      brew link --overwrite formula_name

    To list all files that would be deleted:
      brew link --overwrite --dry-run formula_name

And now it works:

    $ brew link --overwrite suite-sparse
    Linking /usr/local/Cellar/suite-sparse/4.2.0... 87 symlinks created