Cleanup OS specific blocks in PlainDatagramSocketImpl.c to support more unix-like platforms

In source code "jdk\src\solaris\native\java\net\PlainDatagramSocketImpl.c",
there are several macros in the form of:

  #ifdef AF_INET6
  #if defined(__solaris__) || defined(MACOSX)
       // code for solaris and macosx (unix) [1]
  #endif
  #ifdef __linux__
       // code for linux
  #endif
  #else
       // code for non AF_INET6
  #endif /* AF_INET6 */

The code blocks enclosed by the macro are method invocations of
mcast_set_if_by_addr_v6(), mcast_set_if_by_addr_v4(),
mcast_set_loop_v4(), mcast_set_loop_v6(), setHopLimit() and setTTL().

Other unix-like os, i.e. AIX, BSD and some others need the exact calling
sequence coded in block [1]. This CR proposes to a adopt the following
pattern:

  #ifdef AF_INET6
  #ifdef __linux__
       // code for linux
  #else /* __linux__ not defined */
       // code for UNIX
  #endif /* __linux__ */
  #else
       // non AF_INET6
  #endif /* AF_INET6 */

see the full discussion on the net-dev mailing list:
  http://mail.openjdk.java.net/pipermail/net-dev/2012-August/004620.html