Make public function.

PAYLOAD_FORMAT_INDICATOR, CORRELATION_DATA, USER_PROPERTY, CONTENT_TYPE
are now all passed on to subscribing clients from an incoming PUBLISH
only (not from Wills). The other PUBLISH properties are silently
dropped.

For getting the number of bytes needed for writing a property list.

At the moment this only writes a 0 property length, i.e. no properties.
It does mean that output packets are more correct than before.