pf: syncookie support

Import OpenBSD's syncookie support for pf. This feature help pf resist
TCP SYN floods by only creating states once the remote host completes
the TCP handshake rather than when the initial SYN packet is received.

This is accomplished by using the initial sequence numbers to encode a
cookie (hence the name) in the SYN+ACK response and verifying this on
receipt of the client ACK.

Reviewed by:	kbowling
Obtained from:	OpenBSD
MFC after:	1 week
Sponsored by:	Modirum MDPay
Differential Revision:	https://reviews.freebsd.org/D31138

newlib/libc/sys/rtems/include/netinet/tcp.h

@@ -105,6 +105,8 @@ struct tcphdr {
 #define	TCPOPT_FAST_OPEN	34
 #define	   TCPOLEN_FAST_OPEN_EMPTY	2
 
+#define	MAX_TCPOPTLEN		40	/* Absolute maximum TCP options len */
+
 /* Miscellaneous constants */
 #define	MAX_SACK_BLKS	6	/* Max # SACK blocks stored at receiver side */
 #define	TCP_MAX_SACK	4	/* MAX # SACKs sent in any segment */