Add path to realpath for debian, redhat and freebsd

[skreuzer]

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");

[tzz]

This is useful, but it would be even nicer if the built-in filestat() function could resolve the realpath for you.

Recently @jimis added some realpath code in core.git:cf-serverd but I don't know how easy it will be to expose it to filestat().

[tzz]

See https://cfengine.com/dev/issues/4145 for the filestat() feature request