protobuf-c.{c,h}: Implement oneof support

Add a field flag and functions to pack/unpack oneofs
Add logic to parse multiple fields for the same oneof
Add logic for merging submessages that contain oneofs
configure.ac: Bump min protobuf version to 2.6.0 for oneof parsing support
protoc-c: Implement oneofs
t: Add tests for oneoffs

Add test cases for parsing multiple fields of the same oneof from the wire
Add more tests for submessage merging, including oneofs