Change the semantics of GTlsConnection:require-close-notify

Change the semantics of GTlsConnection:require-close-notify

We were combining "allow un-notified closes" and "close without
notifying" into a single property, which meant that it was impossible
to "be liberal in what you accept and conservative in what you send".
Change require-close-notify to only be about the peer behavior, and
make our connections always close-notify properly when closing (while
noting that you can just close the base-io-stream directly if you want
to do an unclean close).