Fix coding style.
Implement some checks per SST spec.
USIDs still need some work.
For future implementation of service/protocol requests.
Update notes in spec.
Move bump_tx_sequence() to state.
Move recovseq initialization.
Implement static_assert() on packet ack mask size.
Don't let ack mask to roll-over to zero in case all previous packets
were seen.
Extend ack mask to 64 packets.
TODO: Check this is not accessed illegally.
Change pktseq type to be full packet_seq_t.
Bump submodules.