Add pb_en/decode_nullterminated() (part of #278)

Nanopb has traditionally supported messages to be terminated with a zero tag.
However, this is not really standard protobuf behaviour, so it makes sense
to separate it into a different function. Because it is a breaking change,
it will happen in 0.4.0 release. But I add the functions here early so that
new code can start using them now.

Also changed the network_server example to use en/decode_delimited(), which
is the more common protobuf method of simple message framing.