added EVTBLK member to FUNC in csoundCore.h and access method in csound.c

As discussed on the dev list, I would like to have access to the evtblk structure that is filled when a table is first created. I've been using a local build with these changes for the past few weeks without any problems. I'm calling csoundTableLength() to check if the table is valid but perhaps there is a more eloquent way of checking the validity of a table. Or perhaps it's best left up to the developer using the API?