This is a first cut at adding proto3 support.

As far as I understand protobuf-c already has pretty much everything
needed once it is built using a new version of protobuf itself.
The only missing thing is that in proto3 all fields are optional and
having to manually set has_foo is inconvenient.

This patch special cases the proto3 syntax files so that structs for the
bytes, enum and primitive fields do not emit the has_ field.

It also adds PROTOBUF_C_LABEL_NONE to the label enum that is used for
proto3 fields. When a fields has this label, the quantifier is not
consulted and instead the field is packed/unpacked depending on
whether it has a value different from NULL/0.