helper/ioutil: silence gcc-8 on strncpy

helper/ioutil: silence gcc-8 on strncpy

Starting from version 8, gcc issues a warning if strncpy could be
able to truncate a string (so without adding the zero-termination
char in destination) by copying exactly "size" char from a source
string not shorter than "size".
Such truncation from strncpy could actually be the desired code
behaviour, but the way to silent gcc only locally (without global
disabling with -Wno-stringop-truncation) through pragma has other
side effects on portability.

In current code, the source string is always "eth0", because has
been checked right above. So this is a false positive from gcc,
being always strlen("eth0") < 16, the sizeof(ifreq.ifr_name).

Silent gcc by decrementing the "size" and remove:
	error: ‘strncpy’ specified bound 16 equals destination
		size [-Werror=stringop-truncation]

This file is only optionally compiled together with ZY1000 driver
with --enable-zy1000 --enable-ioutil. This combination is not
checked in jenkins, so the error passed unnoticed.
Plus, the configure flags above are both deprecated!

Change-Id: I229e66227cfd3513139feeaffa47a6e1ec00767b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5631
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>