Add path to realpath for debian, redhat and freebsd

I would like to add `realpath` to paths.path to work around an issue I have in 3.5.3 where symlinks that do not have the same permissions are not followed. This will make calling the realpath binary more generic across platforms

```
vars:
    "realpath_src" string => execresult("$(paths.path[realpath]) /src", "noshell");
```