sys/socket.h may not always be available (eg. arm-none-eabi toolchain). In the header file we only really need the forward declaration of struct msghdr, so simply do that.
socket.h: Include netinet/in.h instead of arpa/inet.h
In socket.h we are mainly interested in netinet/int.h, since that's the one defining INET_ADDRSTRLEN, struct sockaddr_in, struct in_addr, ec.
Header arpa/inet.h usually defines several functions which use those, like htonl(), inet_ntop(), etc.
See POSIX: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/arpa_inet.h.html#tag_13_03 https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/netinet_in.h.html
