Upgrade protobuf to version 3.5.0.

Pkgsrc changes:
 * Adapt PLIST.
 * After exchange, set MAINTAINER to pkgsrc-users@

Upstream changes:

2017-11-13 version 3.5.0 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
  Planned Future Changes
  * Make C++ implementation C++11 only: we plan to require C++11 to build
    protobuf code starting from 3.6.0 release. Please join this github issue:
    https://github.com/google/protobuf/issues/2780 to provide your feedback.

  General
  * Unknown fields are now preserved in proto3 for most of the language
    implementations for proto3 by default. See the per-language section for
    details.
  * reserve keyword are now supported in enums

  C++
  * Proto3 messages are now preserving unknown fields by default. If you rely on
    unknowns fields being dropped. Please use DiscardUnknownFields() explicitly.
  * Deprecated the unsafe_arena_release_* and unsafe_arena_add_allocated_*
    methods for string fields.
  * Added move constructor and move assignment to RepeatedField,
    RepeatedPtrField and google::protobuf::Any.
  * Added perfect forwarding in Arena::CreateMessage
  * In-progress experimental support for implicit weak fields with lite protos.
    This feature allows the linker to strip out more unused messages and reduce
    binary size.
  * Various performance optimizations.

  Java
  * Proto3 messages are now preserving unknown fields by default. If you'd like
    to drop unknown fields, please use the DiscardUnknownFieldsParser API. For
    example:
      Parser<Foo> parser = DiscardUnknownFieldsParser.wrap(Foo.parser());
      Foo foo = parser.parseFrom(input);
  * Added a new CodedInputStream decoder for Iterable<ByteBuffer> with direct
    ByteBuffers.
  * TextFormat now prints unknown length-delimited fields as messages if
    possible.
  * FieldMaskUtil.merge() no longer creates unnecessary empty messages when a
    message field is unset in both source message and destination message.
  * Various performance optimizations.

  Python
  * Proto3 messages are now preserving unknown fields by default. Use
    message.DiscardUnknownFields() to drop unknown fields.
  * Add FieldDescriptor.file in generated code.
  * Add descriptor pool FindOneofByName in pure python.
  * Change unknown enum values into unknown field set .
  * Add more Python dict/list compatibility for Struct/ListValue.
  * Add utf-8 support for text_format.Merge()/Parse().
  * Support numeric unknown enum values for proto3 JSON format.
  * Add warning for Unexpected end-group tag in cpp extension.

  PHP
  * Proto3 messages are now preserving unknown fields.
  * Provide well known type messages in runtime.
  * Add prefix "PB" to generated class of reserved names.
  * Fixed all conformance tests for encode/decode json in php runtime. C
    extension needs more work.

  Objective-C
  * Fixed some issues around copying of messages with unknown fields and then
    mutating the unknown fields in the copy.

  C#
  * Added unknown field support in JsonParser.
  * Fixed oneof message field merge.
  * Simplify parsing messages from array slices.

  Ruby
  * Unknown fields are now preserved by default.
  * Fixed several bugs for segment fault.

  Javascript
  * Decoder can handle both paced and unpacked data no matter how the proto is
    defined.
  * Decoder now accept long varint for 32 bit integers.


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