string_concat 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 string_concat.
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.
Use enum connection_state instead of int in load_uri,
proxy_uri, get_proxy_worker, and get_proxy_uri. See commit
d18809522e. I hope that satisfies TCC.
