Rolon-relative pathnames

When constructing a DRM (or even a RoleML document), we'll want to use pathnames to refer to other elements in the same Rolon. The best way to do this is with a Rolon-relative pathname, which requires a new notation. I'll note that absolute pathnames can not always be used as the name of the Rolon may not be known.

Rolon-relative pathnames will begin with "@". So for example, the main DSec can be referenced from anywhere in the same rolon by the pathname @/DU/main.

Of course, this required a small change to PHandle. :-(