checkpatch: Add warnings for using strn(cat|cpy)

strn(cat|cpy) has a bad habit of not nul-terminating the destination,
resulting in constructions like

	strncpy(foo, bar, sizeof(foo) - 1);
	foo[sizeof(foo) - 1] = '\0';

However, it is very easy to forget about this behavior and accidentally
leave a string unterminated. This has shown up in some recent coverity
scans [1, 2] (including code recently touched by yours truly).

Fortunately, the guys at OpenBSD came up with strl(cat|cpy), which always
nul-terminate strings. These functions are already in U-Boot, so we should
encourage new code to use them instead of strn(cat|cpy).

[1] https://lists.denx.de/pipermail/u-boot/2021-March/442888.html
[2] https://lists.denx.de/pipermail/u-boot/2021-January/438073.html



Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>