[RFC] vim-patch:7.4.234

Problem:  Can't get the command that was used to start Vim.

Solution: Add v:progpath. (Viktor Kojouharov)

https://code.google.com/p/vim/source/detail?r=d2286df8719d6e99c743e3bf6ac14d1f9debc84d

References https://github.com/neovim/neovim/pull/531