CVE-2018-5704: Prevent some forms of Cross Protocol Scripting attacks

CVE-2018-5704: Prevent some forms of Cross Protocol Scripting attacks

OpenOCD can be targeted by a Cross Protocol Scripting attack from
a web browser running malicious code, such as the following PoC:

var x = new XMLHttpRequest();
x.open("POST", "http://127.0.0.1:4444", true);
x.send("exec xcalc\r\n");

This mitigation should provide some protection from browser-based
attacks and is based on the corresponding fix in Redis:

https://github.com/antirez/redis/blob/8075572207b5aebb1385c4f233f5302544439325/src/networking.c#L1758

Change-Id: Ia96ebe19b74b5805dc228bf7364c7971a90a4581
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reported-by: Josef Gajdusek <atx@atx.name>
Reviewed-on: http://openocd.zylin.com/4335
Tested-by: jenkins
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>