procToArgv calls doesFileExist instead of checking for a slash

`execvp` work by checking for a slash in the path; if there is a slash, it assumes it is a path (absolute or relative), if there is no slash, it looks it up in PATH.

Therefore the current behaviour of `procToArgv` can be unexpected in that if you run `echo` while having a file called `echo` in current directory, it will run your file, while `execvp` will resolve it through PATH.

Open question: will `execv` accept a relative path? If it will, the fix is easy, if it will not, we’ll also have to resolve relative paths.