In prep for FreeBSD 12.0. Will allow i.e. firefox to add support.
Note this doesn't really change the behavior of the client, it still requires the client application to set the TCP_FASTOPEN sockopt prior to calling sendto. This is similar to other OSes like Linux where you have to send with MSG_FASTOPEN to opt in to TFO, or OS X where you use a new connectx() system call.
This matches Linux as a capable TFO client by default https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0d41cca490c274352211efac50e9598d39a9dc80