This goes through the extentsions the client requested, selects the ones
that we support at the server, and then further calls back to the appropriate
protocol callback in user code to check it's OK to actually use that
extension in this context. If it is (callback unhandled == it is) then
it's added to the list of extensions sent back to the client.
Accepted extensions are also added to the connection's active extension
list and the private "user" memory allocation for the extension context is
done and bound to the connection.
Signed-off-by: Andy Green <andy@warmcat.com>
c511482c78