banned.h: mark strcat() as banned

The strcat() function has all of the same overflow problems as strcpy(). And as a bonus, it's easy to end up accidentally quadratic, as each subsequent call has to walk through the existing string. The last strcat() call went away in f063d38b80 (daemon: use cld->env_array when re-spawning, 2015-09-24). In general, strcat() can be replaced either with a dynamic string (strbuf or xstrfmt), or with xsnprintf if you know the length is bounded. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>