First step towards unifying triggers into rpmScript API

First step towards unifying triggers into rpmScript API

- Add a lower level script creation function to deal with the
  body expansions and such, use it for triggers as well.
- This is still fairly ugly but its something that can be reasonably
  backported to 4.9.x which needs this too, as currently triggers
  are forgetting to set script->descr, causing "(null) failure" messages
  on glibc and on others, in would just crash on trigger failure
  and/or in debug verbosity level.