Exposed the bNodeSocket->limit value in RNA, so it can be redefined by pynodes. All sockets will have strict n-to-1 connectivity by default, which is fine for data sockets, but things like geometry nodes with a more data-flow based connectivity for certain socket types will need to change this.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55898 954f8c5b-7b00-dc11-b283-0030488c597c