Implement AccECN option (w/ early assigned option number)

[rscheff]

Summary

This Diff builds upon D21011. A simplistic reflector for
the received CE, ECT1 and ECT0 marked data bytes in the
AccECN TCP option - which is sent on every ACK, in full.
Only when space is needed for SACK, an abbreviated option
is constructed and sent (if possible).

Potential enhancements: Track which counters changed, and
send only changed counters repeatedly (e.g. 3 times at least)

[rscheff]

rebase to main

[rscheff]

validating this diff against accecn packetdrill scripts from Linux (Ilpo). Once done, will bring this up again.

[rscheff]

• move ecn sysctls to tcp_ecn.c
• go with AccECN options from a compile time option to a global setting
• document the new sysctl option
• remove outdated comment

[rscheff]

• rebase to main

[rscheff]

• ensure control packets have accecn opt, if enabled

[rscheff]

• move ecn sysctls to tcp_ecn.c
• go with AccECN options from a compile time option to a global setting
• document the new sysctl option
• remove outdated comment
• add logging, and make syncache response honor most recent incoming SYN fields
• add logging
• more logging
• ensure control packets have accecn opt, if enabled
• fix packt_this_ack bug
• bump man date
• received accecn counter into tcpinfo
• add more tcpinfo counters
• make accecn option counters a struct within tcpcb, which can also be locally allocated (syncache, prior of tcpcb)
• initialize to_ae properly
• reference properly to counters
• bump man page date
• remove all logging