make .alloc and .free methods of ProtobufCAllocator callable again

it turned out to be a bad idea to not export the default allocator's
methods via the ProtobufCAllocator's function pointers. there is at
least one user (protobuf-c-rpc) that makes allocations by directly
invoking those methods.
ChangeLog: the entries about the rpc code have been moved to protobuf-c-rpc's ChangeLog
ChangeLog: configure --disable-protoc has been restored
ChangeLog: mention google compat symlink and protobuf-c-rpc splitout