Add socket interface.

libc/Makefile

libc/arpa/inet/inet_addr.cpp

libc/arpa/inet/inet_ntoa.cpp

libc/arpa/inet/inet_ntop.cpp

libc/arpa/inet/inet_pton.cpp

libc/decl/in_addr_t.h

libc/decl/in_port_t.h

libc/decl/sa_family_t.h

libc/decl/socklen_t.h

libc/include/arpa/inet.h

libc/include/netdb.h

libc/include/netinet/in.h

libc/include/sys/socket.h

libc/include/sys/un.h

libc/netdb/endhostent.cpp

libc/netdb/endnetent.cpp

libc/netdb/endprotoent.cpp

libc/netdb/endservent.cpp

libc/netdb/freeaddrinfo.cpp

libc/netdb/gai_strerror.cpp