tfo-go caches the "unsupported" status when fallback mode is enabled.
In other words, if the hysteria server is started with
net.ipv4.tcp_fastopen=0 and it fails once, the tfo will not be enabled
until it is restarted, even if the user later sets sysctl
net.ipv4.tcp_fastopen=3.
d8c61c59d7