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
Update os_can_exe() unit tests.

patch 235 added a new argument to os_can_exe(), the path of the
executable. Update the tests to verify the output as well.