conda.install.symlink_conda: Check for symlink targets

Conda currently assumes a system-wide install:

    $ pip install --user conda
    $ command -v conda
    /home/me/.local/bin/conda
    $ ls -l ~/envs/foo/bin/conda
    lrwxrwxrwx 1 ... /home/me/envs/foo/bin/conda -> /usr/bin/conda

I'm not sure if it's better to adjust:

    actions[SYMLINK_CONDA] = [config.root_dir]

or:

    root_dir = abspath(expanduser(os.getenv('CONDA_ROOT',
                                            rc.get('root_dir', sys.prefix))))

but regardless of how we get things working, we'll still want this
check to make sure we're not broken.