Fixes vimrc for root via poor-man's abspath

1. Tack the link's target onto the naïve "dirname":
/full/path/to/.vimrc => /full/path/to//absolute/real/vim/vimrc

2. Fix up absolute path treated like a relative path:
/full/path/to//absolute/real/vim/vimrc => /absolute/real/vim/vimrc