update sshd_core_cluster

Main changes include incresing external availability for the clustered
sshd_analyzer_cluster policy and cleaning up the old session identifier
style.

Fixed up some messy code

Fixed print_sid() so that it returns value always.

Add register_cid() to allow the sftp (and any other subsystem) to be
able to look up cid of the calling session based on sid and ppid.