Common type holding an address

This modifies all transports to use a new common address type, struct
address. This address is stored in a ptp_message for all received messages.

For sending, the "default" address is used with the default sending
functions, transport_send and transport_peer. The default address depends on
the transport; it's supposed to be the multicast address assigned by the
transport specification.

Later, a new transport_sendto function will be implemented that sends to the
address contained in the passed ptp_message.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Implement transport_sendto

Also, document transport_send, transport_peer and transport_sendto usage.

Signed-off-by: Jiri Benc <jbenc@redhat.com>