Overhaul how custom Node actions and status strings are handled

Custom NodeStatus and NodeAction/Activity strings are now displayed pretty
much everywhere possible (except the node utility, since it doesn't read
any configuration files). This fixes issue #618, finally.

If you have custom (non-blank) NodeAction* strings in your text.dat/text.ini
file, or you change these strings during runtime (e.g. using bbs.replace_text)
this will impact you:
- do not include the user name or any other fields normally included in
  printed node status in your string - just the customized node action and
  really, you should only use/override these strings for dynamically changing
  node status (i.e. you're using NodeActionCustom and you're setting different
  custom strings depending on current node activity). Consider using the new
  NodeActivity* text.dat strings instead if your customized string doesn't
  change during run-time.
- don't use the %s/%u/%d specifiers any more - use @-codes instead, if needed

If you want static/permanent node action strings, set new new NodeActivity*
strings in your text.dat or (preferrbaly) text.ini file instead.
This means the node actions can now be localized for other languagues!

I'm still using the node.exb file (I did consider alternatives, e.g.
node.ini or node*/activity.asc), but decided to postpone such a change.
The node.exb file is still used, but the strings represent just the custom
(extended) node *activity* and not the entire node status string.

Use of Ctrl-A codes needs to be considered/accounted-for (currently isn't).