[RFC] vim-patch:7.4.235

> Problem:    It is not easy to get the full path of a command.
> Solution:   Add the exepath() function.

> https://code.google.com/p/vim/source/detail?r=5ab2946f7ce560985830fbc3c453bb0f7a01f385

I think this should remain a WIP until #938 gets merged, but I thought it would be better to put this up rather than just sit on it. A few things might change before [RDY] anyway.
Use `///` comments style.