API: libcrmcommon: drop struct pe_node_shared_s unseen member

Internally, use a new node flag (with the sense flipped) instead.

Flipping the sense is not a problem. Previously, pe_create_node() allocated the
new node (with unseen implicitly FALSE), then handle_startup_fencing() would
immediately set unseen to TRUE for cluster nodes. Now, pe_create_node()
implicitly creates the node as unseen, which is the same effect for cluster
nodes.

This breaks API backward compatibility.