net: tcp: Add optional TIME_WAIT support

The RFC requires we honor the 2MSL TIME_WAIT timeout, support for
which was just removed with the FIN cleanup.  Add it back, but make it
optional (proper sequence number and ephemeral port randomization
makes true collisions a birthday problem in a ~80 bit space!).

Change-Id: I176c6250f43bba0c914da1ee7f0136dcb1008046
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>