This fixes a couple of skyzaller crashes. Most
of them have to do with TFO. Even the default stack
had one of the issues:

1. We need to make sure for rack that we don't advance snd_nxt beyond iss when we are not doing fast open. We otherwise can get a bunch of SYN's sent out incorrectly with the seq number advancing.
2. When we complete the 3-way handshake we should not ever append to reassembly if the tlen is 0, if TFO is enabled prior to this fix we could still call the reasemmbly. Note this effects all three stacks.
3. Rack like its cousin BBR should track if a SYN is on a send map entry.
4. Both bbr and rack need to only consider len incremented on a SYN if the starting seq is iss, otherwise we don't increment len which may mean we return without adding a sendmap entry.

This work was done in collaberation with Michael Tuexen, thanks for
all the testing!
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D25000