src/frontend/postcoms.c, bugfix, write/save an xspice `event' node

In function EVTfindvec() XSPICE can create
  a `dvec' with a non NULL `v_scale' slot.
This special scale with name "step"
  differs from the current plot `pl_scale' slot
  and triggered ancient buggy code here.
cleanup, signature of CKTacDump() and CKTdump(), void* --> runDesc*
src/frontend/postcoms.c, cleanup, unused variable `appendwrite'