RFR 8004675: Inet6Address.getHostAddress should use string scope identifier where available

[Chris Hegarty]

Inet6Address.getHostAddress() is specified to return the IP address 
string in textual presentation, followed by a '%' character and the 
scope identifier. This scope identifier can be either a numeric value or 
a string, depending on how the instance was created (if it was created 
with a scoped interface).

This change proposes to remove the boolean field, 'scope_ifname_set', 
since it is not always correctly set when the instance contains a valid 
scoped interface. For example, when iterating over the 
NetworkInterface's on the system. 'scope_ifname_set' was never accessed 
from native code, so it can simply be removed.

http://cr.openjdk.java.net/~chegar/8004675/webrev.00/webrev/

-Chris.