Add path to realpath for debian, redhat and freebsd

The change in cfengine 3.5.3 where symlinks that point to files or directories that do not have the same permissions broke a few of my agents that have to copy files from a symlink / that points to another symlink that points to the release code. I worked around this by using realpath to get the real directory.

```
vars:
  "realpath_src" string => execresult("$(paths.path[realpath]) /src", "noshell");
```

I would like to add the path to the `realpath` binary on several platforms into the paths.cf library so my agents will be more generic across platforms.