tcp: improve sending of SYN-cookies

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.

Reviewed by: rscheff, cc, Peter Lei, Nick Banks
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52225