4440. [func] Enable TCP fast open support when available on the

server side. [RT #42866]

(cherry picked from commit a977bc4c8e)

doc/misc/tcp-fast-open

@@ -0,0 +1,27 @@
+Copyright (C) 2016  Internet Systems Consortium, Inc. ("ISC")
+
+This Source Code Form is subject to the terms of the Mozilla Public
+License, v. 2.0. If a copy of the MPL was not distributed with this
+file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Some systems (Linux, FreeBSD, OS X/macOS and Windows 10) support
+the TCP Fast Open (RFC 7413) mechanism in their recent versions.
+
+BIND 9 supports this on the server side.
+
+When the TCP_FASTOPEN socket option is defined after the listen()
+system call the socket code in the libisc set the option with
+the half of the listen backlog (so the fast open maximum queue length
+is the half of the pending connection queue length).
+Any failure is logged and ignored.
+
+System specific notes:
+ - FreeBSD doesn't interpret the argument as a queue length but
+  only as an on/off switch.
+
+ - Apple OS X/macOS allows only 0 or 1 so the code puts 1 for this system.
+
+ - Windows 10 uses a 0/1 char flag and perhaps as not yet server support?
+
+ - the only other system known to support this is Linux.
+