Don't return duplicate matches in xolox#misc#path#which()
Make xolox#misc#os#find_vim() configurable

xolox/vim-easytags#54 shows that it can be useful to override the default
logic of xolox#misc#os#find_vim(), even if it is to work around other
problems:

  https://github.com/xolox/vim-easytags/pull/54