I don't know why it wasn't indented properly, but we may as well fix
it now.

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.

The dir-spec.txt specifies it to be "hsdir_interval" (underscore).

Fixes #24262

Signed-off-by: David Goulet <dgoulet@torproject.org>