Allow providing a node to string function that will be activated
when the root node pformat routine needs to format a node into
a string (for viewing purposes); this allows for subclasses or
other users of nodes to provide there own function that they can
specialize as they see fit (without duplicating the pformat
routine itself).
To start use it to print out the node link target in the fake
in-memory filesystem when a link is encountered.
Change-Id: I33b8f1ec8d30d72ae35971e35f5edd84a9145db7
de68bc2d72