8034182: Misc. warnings in java.net code

[Alan Bateman]

This is drive-by fix to a number of native code warnings in the 
networking code.

In NET_SockaddrToInetAddress then CHECK_NULL_RETURN is used to check the 
return value from setInet6Address_ipaddress but the return is an int 
rather than a pointer. In PlainDatagramSocketImpl's mcast_join_leave 
then the issue is using a jbyte* when a char* is expected. In 
Inet6AddressImpl.c then the warning is code before declaration (the 
SunStudio compilers complain about that).

While looking at these warnings then I noticed that the 
setInet6Address_XXX functions (and one of the get functions) return a 
jboolean but are declared to return an int. While not wrong, it's not 
obvious so I changed these functions to return a jboolean.

The webrev with the proposed changes is here:

http://cr.openjdk.java.net/~alanb/8034182/webrev/

-Alan.