nm_utils_ip4_prefix_to_netmask() is public API of libnm.
As we also want to have this function at a few places where
we don't have libnm, we have an internal variant
_nm_utils_ip4_prefix_to_netmask().
Use the internal variant consistently and everywhere.
Review and replace usages of the two nm_connection_to_dbus() flags
marked deprecated in commit 84648e562c98 ('libnm: Refactor
NM_CONNECTION_SERIALIZE_* flags'):
NM_CONNECTION_SERIALIZE_NO_SECRETS and
NM_CONNECTION_SERIALIZE_ONLY_SECRETS.
