hammer2 - kernel cluster messaging support API work

hammer2 - kernel cluster messaging support API work

* Rework the API significantly

* Add circuit tracking and sniffing features

* Add flags to automate the LNK_CONN, LNK_SPAN, and LNK_CIRC state machines.

* Misc improvemenets.

• [DBH] sys/kern/kern_dmsg.c
• [DBH] sys/kern/subr_diskiocom.c
• [DBH] sys/sys/dmsg.h