Greatly reduce the number of #ifdefs supporting the TCP_RFC7413 kernel option. The conditional compilation support is now centralized in tcp_fastopen.h and tcp_var.h. This doesn't provide the minimum theoretical footprint when TCP_RFC7413 is disabled, but nearly all the TFO code should wind up being removed by the optimizer, the additional footprint in the syncache entries is a single pointer, and the additional overhead in the tcpcb is at the end of the structure.
This enables TCP_RFC7413 by default in amd64 and arm64 GENERIC.
This revision depends on D14047.