bsd: add comparison operators to tcp_seq

Add comparison operators that use modulo arithmetic to order sequence
numbers, and use them to replace SEQ_LT() and friends, increasing code
readability.

As a consequence std::min() and std::max() can be used instead of SEQ_MIN()
and SEQ_MAX().

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>