switch from gethostbyname to getaddrinfo on POSIX systems

gethostbyname may return any combination of IPv4 and IPv6 addresses,
and it's not safe to assume the first address is IPv4, which is all
our code is currently prepared to handle.  In contrast, getaddrinfo
allows us to specify whether we want IPv4, IPv6, or both.

We should eventually make this switch on Windows as well, but the
status of getaddrinfo in Windows 2000 is not clear, and MinGW's
ws2tcpip.h only declares it for XP and above.

This commit also adds InetAddress.getByName for explicit DNS lookups.

classpath/java/net/Socket.java

@@ -10,6 +10,10 @@
 
 package java.net;
 
+import java.io.IOException;
+
 public abstract class Socket {
+  public static native void init() throws IOException;
+
   public abstract void setTcpNoDelay(boolean on) throws SocketException;
 }