Ensure that when the `sysctl`-variable `net.inet.tcp.syncookies_only` is non zero, SYN-cookies are sent and no SYN-cache entry is added to the SYN-cache. In particular, this behavior should not depend on the value of the `sysctl`-variable `net.inet.tcp.syncookies`, which controls whether SYN cookies are used in combination with the SYN-cache to deal with bucket overflows.
Also ensure that `tcps_sc_completed` does not include TCP connections established via a SYN-cookie.
While there, make `V_tcp_syncookies` and `V_tcp_syncookiesonly` `bool` instead of `int`, since they are used as boolean variables.