path: learn save_absolute_path().
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 test
Merge pull request #953 from splinterofchaos/patch235-exepath

vim-patch:7.4.235