This updates the join node to use the new multi-input socket from D10067. The change just requires slightly changing the arguments of the exec function, and changing the socket input list. Note that this commit does not contain the UI changes, it only allows for more input links, and combines the two sockets into one. The UI changes will come next. Reviewed By: Hans Goudey Differential Revision: https://developer.blender.org/D10069