vp2buf
b.len = vp2buf(myself, &b.buf, b.len, fmt, ap);
pb.len = vp2buf(msg, &pb.buf, pb.len, fmt, ap);
b->len = vp2buf(msg, &b->buf, b->len, fmt, ap);
int vp2buf(const char *msg, char **buf, int buflen, const char *fmt,