Split ecpg_execute() in constituent parts

Split the rather long ecpg_execute() function into ecpg_build_params(),
ecpg_autostart_transaction(), a smaller ecpg_execute() and
ecpg_process_output().  There is no user-visible change here, only code
reorganization to support future patches.

Author: Zoltán Böszörményi

Reviewed by Antonin Houska.  Larger, older versions of this patch were
reviewed by Noah Misch and Michael Meskes.