Make the DataDirectory option immutable.

By convention, the torrc options that the user sets are
unchangeable.  If we need to change them, we should be using a copy
that's stored in another field

To avoid trouble, I'm keeping DataDirectory as the name for the
field that the rest of Tor uses, and using DataDirectory_option for
the confparse-controlled field.

This commit also modernizes some older string handling code in the
DataDirectory normalization function.