Add an API for creating real rpmScript items out of triggers, use it

Add an API for creating real rpmScript items out of triggers, use it

- Bury rpmScriptNew() into being internal helper in rpmscript -
  triggers and other scripts differ quite a bit in how their data
  is laid out in the header, especially args need "special attention".
- Besides cleaning up things in the psm side, this technically makes
  trigger scripts runnable without having a header at hand. Of course
  currently trigger scripts are currently created and destroyed
  on the spot from headers so this is of academic interest...