tcp: improve tcp_ecn by using switch statements and bit-shifting

[rscheff]

Summary

In order to improve readability, use switch statements to
differentiate between the different ECN signallings.

Also, access the ACE field by bit-shifting instead checking
each flag individually.

No change in behaviour.

[glebius]

P.S. There are so many sysctl toggles that ask for SYSCTL_INT_MINMAX() macro, to avoid coding out yet another value checking function but to block user from entering invalid value.

sys/netinet/tcp_ecn.c

103

AFAIK, these should be one liners per style(9).

125

Not sure it is documented, but usual habit is to leave default: as the last case.

[rscheff]

• remove default case, as falling through and returning is the correct action.

[tuexen]

sys/netinet/tcp_ecn.c

229

The indentation looks strange here...

[rscheff]

• remove default: when no special action is necessary
• place statemens after case: in new line
• add /* FALLTHROUGH */ comments per style(9)