TCP complete end status work.

The ending of a connection can tell us a lot about what happened i.e. did
it fail to setup, did it timeout, was it a normal close. Often times this is
useful information to help analyze and debug issues. Rack has had
end status for some time but the base stack as not. Lets go a ahead
and add in the missing bits to populate the end status.

Reviewed by: tuexen, rscheff
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D36712