APNs can only contain alphanumeric characters, '.', and '-'. To be
helpful we strip spaces off before setting the APN internally so that
previously (and incorrectly) valid APNs don't cause the whole
connection to fail validation and thus disappear. The only case seen
in the wild was a Pelephone IL APN which erroneously had a trailing
space in the mobile broadband provider database. Bad characters
cause the connection to fail with vague error messages about being
unable to activate the PDP context during PPP negotiation.
Ensure it still works correctly if something tries to set the
'addresses' property using the old GType. Also make sure that
the various IP6 address comparison operations and string conversion
functions handle the gateway.
