Extract ipv4 address from dns answer packet
#include <dns.h>
int dns_ip4_packet(stralloc* out,const char* buf,
unsigned int len);
dns_ip4_packet is a low-level component of dns_ip4, designed to support asynchronous DNS lookups. It reads a DNS packet of length len from buf, extracts IP addresses from the answer section of the packet, puts the addresses into out, and returns 0 or -1 the same way as dns_ip4.