6706208: (cs) CharsetProvider permission check cleanup

[Alan Bateman]

This is a small clean-up to the permission check in CharsetProvider and 
SelectorProvider to make it more robust. Note that this is not a 
security issue, the permission check is really only interesting for the 
default provider and it requires opt-in configuration to change it. The 
webrev with the change is here:

http://cr.openjdk.java.net/~alanb/6706208/webrev/

Thanks,
Alan.