straconcat reads the args with va_arg(ap, const unsigned char *),
and the NULL macro may have the wrong type (e.g. int).
Many places pass string literals of type char * to straconcat. This
is in principle also a violation, but I'm ignoring it for now because
if it becomes a problem with some C implementation, then so will the
use of unsigned char * with printf "%s", which is so widespread in
ELinks that I'm not going to try fixing it now.
It will grab at the first fragment of the cache entry and try to detect the
content-type by looking for valid HTML. It is very stupid for now, simply
searching for "<html>", which may be bogus in certain circumstances. And I
am not sure if this is better left out and up to the scripting backends,
e.g. SMJS can now modify the cache entry.
A feable fix for bug 396.
Its return value is now an enum that lets callers know whether an
error occurred. However, this commit changes the callers only
minimally, so they do not yet check the return value.
