Branches/protobuf c rpc split out

i'd like to split protobuf-c into two repositories: "protobuf-c", containing the "core" parts, the protoc-c compiler and the stripped down libprotobuf-c which now contains just the protobuf encoding/decoding support library; and a new repository "protobuf-c-rpc", containing just the libprotobuf-c-rpc library.

i've prepared a fork of protobuf-c containing just the RPC components, see https://github.com/protobuf-c/protobuf-c-rpc. to make it clearer which identifiers belong to "protobuf-c" or "protobuf-c-rpc" i've renamed a lot of identifiers in protobuf-c-rpc to include an "rpc" or "RPC". "protobuf-c-rpc" is passing its CI builds, but i believe there is a certain amount of RPC example code that is not compiled by the build system that may have to be updated due to the API break.

this branch of "protobuf-c" contains commits that remove the now redundant bits that are contained in "protobuf-c-rpc".

@lipnitsk, are you ok with this approach? it will mean code that uses protobuf-c-rpc now needs to depend on both protobuf-c and protobuf-c-rpc, and RPC code will have to be updated where identifiers in the RPC library and headers have been renamed. it also means the issues reported against the RPC code in the protobuf-c repository will somehow have to be migrated into the protobuf-c-rpc repository.