extract TXT records from DNS answer packet
int dns_txt_packet(stralloc* out,const char* buf,
unsigned int len);
dns_txt_packet is a low-level component of dns_txt, designed to support
asynchronous DNS lookups. It reads a DNS packet of length len from buf,
extracts the TXT records from the answer section of the packet, puts the
result into out, and returns 0 or -1 the same way as dns_txt.