core-util: Attempt to make runtime paths smaller to avoid 108 char limit.
When the runtime path gets long (which can happen on some NFS
mounts where $HOME is not just /home/$USER), it can grow
longer the 108 char limit imposed by sockaddr_un.sun_path.
This just calls realpath which should ultimately point into
/tmp in most cases and result in a much smaller path.
Only do this when we are adding on a name component to the
runtime path so creating the actual symlink will still get
the original, long name, but this shouldn't be a problem
as it never goes into the sockaddr_un.sun_path.