protoc-c: add Options class and add a no_name_mangling option

This addresses issue #104 and ports a fix from my own repo of protobuf-c to the main repo. As the issue description points out, we need to make sure this is what the project needs, since using the new option will not work in all circumstances, but may be useful in certain cases.

I don't mind at all if this pull request is not pulled into master but want to throw it out there just in case somebody might find it useful. The default compiler behavior does not change and with the new Options class the compiler could be extended further, if needed.