Expose internal and external port numbers in the environment

Now that complex port mapping syntax is implement we can't rely on the
assumption that the external port number matches the internal one.
Services need to know what internal, exposed port they need to bind to,
which might be different from the external port addressable from outside
the container. These internal port numbers are placed in the environment
in the same way as the other port numbers but using _INTERNAL_PORT
instead of _PORT.

The get_specific_port() guest util function still returns the external
port, and a new guest util function, get_specific_exposed_port(), is
added that returns the internal port. The existing get_port() method
returns the internal port since it's commonly used inside container
start scripts to configure the service that will be executed.

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>