helper/options.c: fail if unexpected cmdline arguments are present

Previously openocd would silently ignore any incorrect arguments.

Change-Id: Ibb40b57b8a9e07d191215486f3b3c4920a9963c7
Signed-off-by: Girts Folkmanis <opensource@girts.me>
Reviewed-on: http://openocd.zylin.com/3879


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>