[logging] assign blocks to different logging subsystems

• drop KNET_SUB_PMTUD that was unused
• add KNET_SUB_TRANSPORT_T that was missing
• switch from "common" to "unknown" for odd logging
• fix up code around to allow holes in the structs
• fix up test suite

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>