net: tcp: Clean up FIN handling

The connection close paths were a little tangle. The use of separate
callbacks for "active" and "passive" close obscured the fundamentally
symmetric operation of those modes and made it hard to check sequence
numbers for validation (they didn't).  Similarly the use of the
official TCP states missed some details we need, like the distinction
between having "queued" a FIN packet for transmission and the state
reached when it's actually transmitted.

Remove the state-specific callbacks (which actually had very little to
do) and just rely on the existing packet queuing and generic sequence
number handling in tcp_established().  A few new state bits in the
net_tcp struct help us track current state in a way that doesn't fall
over the asymmetry of the TCP state diagram.  We can also junk the
FIN-specific timer and just use the same retransmit timer we do for
data packets (though long term we should investigate choosing
different timeouts by state).

Change-Id: I09951b848c63fefefce33962ee6cff6a09b4ca50
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>